PLOS ONE RESEARCH ARTICLE The COVID-19 Sentinel Schools Network of Catalonia (CSSNC) project: Associated factors to prevalence and incidence of SARS-CoV-2 infection in educational settings during the 2020–2021 academic year Fabiana Ganem ID1,2,3☯, Anna Bordas1,3☯, Cinta Folch ID1,3,4*, Lucia Alonso1,3, a1111111111 Marcos Montoro-Fernandez1, Andreu Colom-Cadena1,3, Ariadna Mas5, a1111111111 Jacobo Mendioroz6, Laia Asso7, Andres Anton8, Tomàs Pumarola8, Maria a1111111111 Victoria González9, Ignacio Blanco9, Pere Soler-Palacı́n10, Antoni Soriano-Arandes ID10, a1111111111 Jordi Casabona1,2,3,4, on behalf of Sentinel School Network Study Group of Catalonia¶ a1111111111 1 Centre of Epidemiological Studies on Sexually Transmitted Infections and AIDS of Catalonia (CEEISCAT), Health Department, Government of Catalonia, Badalona, Spain, 2 Departament de Pediatria, d’Obstetrı́cia i Ginecologia i de Medicina Preventiva i de Salut Publica, Universitat Autònoma de Barcelona, Bellaterra, Spain, 3 Institut d’Investigació Germans Trias i Pujol (IGTP), Badalona, Spain, 4 Spanish Consortium for Research on Epidemiology and Public Health (CIBERESP), Instituto de Salud Carlos III, Madrid, Spain, OPEN ACCESS 5 Direcció Assistencial d’Atenció Primària i Comunitària, Institut Català de la Salut, Barcelona, Catalonia, Spain, 6 Subdirecció general de Vigilància i Resposta a Emergències de l’Agència de Salut Pública de Citation: Ganem F, Bordas A, Folch C, Alonso L, Catalunya, Departament de Salut, Catalonia, Spain, 7 Agència de Salut Pública de Catalunya (ASPCAT), Montoro-Fernandez M, Colom-Cadena A, et al. Departament de Salut, Generalitat de Catalunya, Catalonia, Spain, 8 Microbiology Department, Vall (2022) The COVID-19 Sentinel Schools Network of d’Hebron Institut de Recerca (VHIR), Vall d’Hebron Hospital Universitari, Vall d’Hebron Barcelona Hospital Catalonia (CSSNC) project: Associated factors to Campus, Universitat Autònoma de Barcelona, Barcelona, Catalonia, Spain, 9 Microbiology Department, prevalence and incidence of SARS-CoV-2 infection Laboratori Clı́nic Metropolitana Nord, Hospital Universitari Germans Trias i Pujol, Institut Català de la Salut, Institut D’Investigació en Ciències de La Salut Germans Trias i Pujol (IGTP), Badalona, Catalonia, Spain, in educational settings during the 2020–2021 10 Pediatric Infectious Diseases and Immunodeficiencies Unit, Hospital Universitari Vall d’Hebron, Vall academic year. PLoS ONE 17(11): e0277764. d’Hebron Institut de Recerca, Universitat Autònoma de Barcelona, Barcelona, Catalonia, Spain https://doi.org/10.1371/journal.pone.0277764 Editor: Omnia Samir El Seifi, Zagazig University ☯ These authors contributed equally to this work. ¶ Membership of the Sentinel School Network Study Group of Catalonia is provided in Acknowledgments. Faculty of Human Medicine, EGYPT * cfolch@iconcologia.net Received: April 5, 2022 Accepted: November 3, 2022 Published: November 17, 2022 Abstract Copyright: © 2022 Ganem et al. This is an open The Sentinel Schools project was designed to monitor and evaluate the epidemiology of access article distributed under the terms of the COVID-19 in Catalonia, gathering evidence for health and education policies to inform the Creative Commons Attribution License, which development of health protocols and public health interventions to control of SARS-CoV-2 permits unrestricted use, distribution, and reproduction in any medium, provided the original infection in schools. The aim of this study was to estimate the prevalence and incidence of author and source are credited. SARS-CoV-2 infections and to identify their determinants among students and staff during Data Availability Statement: All relevant data are February to June in the academic year 2020–2021. We performed two complementary stud- within the paper. ies, a cross-sectional and a longitudinal component, using a questionnaire to collect nominal Funding: This work was supported by the Health data and testing for SARS-CoV-2 detection. We describe the results and perform a univari- Department of the Government of Catalonia with ate and multivariate analysis. The initial crude seroprevalence was 14.8% (95% CI: 13.1– no grant number. The funders had no role in study 16.5) and 22% (95% CI: 18.3–25.8) for students and staff respectively, and the active infec- design, data collection and analysis, decision to publish, or preparation of the manuscript. FG, AB, tion prevalence was 0.7% (95% CI: 0.3–1) and 1.1% (95% CI: 0.1–2). The overall incidence LG and ACC received a salary from the for persons at risk was 2.73 per 100 person-month and 2.89 and 2.34 per 100 person-month abovementioned funder. PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 1 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Competing interests: The authors have declared for students and staff, respectively. Socioeconomic, self-reported knowledge, risk percep- that no competing interests exist. tions and contact pattern variables were positively associated with the outcome while sani- tary measure compliance was negatively associated, the same significance trend was observed in multivariate analysis. In the longitudinal component, epidemiological close con- tact with SARS-CoV-2 infection was a risk factor for SARS-CoV-2 infection while the highest socioeconomic status level was protective as was compliance with sanitary measures. The small number of active cases detected in these schools suggests a low transmission among children in school and the efficacy of public health measures implemented, at least in the epidemiological scenario of the study period. The major contribution of this study was to pro- vide results and evidence that help analyze the transmission dynamic of SARS-CoV-2 and evaluate the associations between sanitary protocols implemented, and measures to avoid SARS-CoV-2 spread in schools. Introduction The Coronavirus Disease 19 (COVID-19) outbreak began in Wuhan, China in December 2019 and rapidly became an international public health emergency. As of March 2022, there had been more than 470 million cases and 6 million deaths globally [1]. The first case of COVID-19 in Spain was confirmed on January 31, 2020, and in Catalonia on February 25 [2]. Until March 2022, the Catalan region had registered more than 2 million accumulated cases and more than 26,000 deaths [3]. At the beginning of the pandemic, in March 2020, it was estimated that 107 countries and 862 million children and young people were affected by the closure of schools, one of the pub- lic health measures aiming to reduce the transmission of SARS-CoV-2 [4]. However, this num- ber increased to 1.57 billion students worldwide over the following months [5]. Many governments chose to close schools in response to the pandemic because it has previously been shown to be an effective non-pharmacological prevention measure in the control of other virus spread like influenza [4, 6] where children have had a significantly contribution [7–9]. Nevertheless, at the beginning of the current pandemic, data on the prevalence of COVID-19 in children was scarce due to low testing of the pediatric population [10] and the fact that parameters and evidence about COVID-19 occurrence in adults could not be extrapolated to children [11]. A great deal of effort was made to resolve this question. Since the beginning of the pandemic, the contribution of children in the virus spread has been discussed [12]. People aged from 0 to 14 had a lower risk of SARS-CoV-2 infection com- pared to those of 15 to 64, additionally, among the infected, older people had more severe out- comes and reported higher mortality rates [4]. Despite estimates based in household secondary attack rates may be influenced due to sev- eral factors such as contact patterns, increased exposure, and symptomatic surveillance, which depends on the sensitivity of case detection, even several serological studies estimate highest prevalence among adults under 35 years [13, 14], it is known that children are not the main source of spread of the SARS.CoV-2 virus and therefore interventions based on this public can have an impact below expected [15, 16]. The impact of school closure could also cause social, economic and health problems, with emotional costs for children and young people considering the interruptions to other areas of PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 2 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year activity in schools such as, nutrition, mental health, safety, and social assistance services [6, 13, 14, 17–19]. In Catalonia (7,739,758 inhabitants) the closure of the 5,492 schools on 13 March 2020 affected 1,582,478 students and 116,999 teaching staff. The schools were reopened on 14 Sep- tember 2020, immediately after the school vacations, between June and August, remaining closed, therefore, for six months [20]. The current schools’ guidelines were developed by the Government of Catalonia based on SARS-CoV-2 indicators. They include early detection and isolation as well implementation of public health measures as natural ventilation of classrooms, stable coexistence groups (SCG or bubble groups) and targeted screening [21, 22], also monitoring COVID-19 risk factors, deter- minants, transmission dynamics, preventive measures compliance and outbreaks in the school to provide evidence to improve the safety of schools preventing further closures and their impact [4, 18, 23]. The COVID-19 Sentinel Schools Network of Catalonia (CSSNC) is a part of the COVID- 19 monitoring and evaluation plan from the Health Department of Catalonia. The main objective is to monitor and evaluate the epidemiological situation of COVID-19 and its determinants in the educational setting, to gather evidence for the health policies aimed at the prevention and control of SARS-CoV-2 infection in schools and as a platform for other applied research projects. Currently, in 2022, the CSSNC includes 23 schools with 4,221 chil- dren and 1,140 staff from all over Catalonia, the study protocol has been previously pub- lished [24]. The aims of the CSSNC project (www.escolessentinella.org) include: the monitoring of bio- logical markers; knowledge, attitudes and behaviors towards SARS-CoV-2 preventive mea- sures; the identification of both facilitators and barriers to their implementation; and the monitoring of environmental indicators such as CO2, all together using a participatory research approach [24]. In this paper, to answer the question about the occurrence of COVID-19 among school- aged population, the main objectives are to estimate the prevalence and incidence of SARS- CoV-2 exposure and infections and to identify potential associated factors associated to them, among students and staff of the CSSNC during the academic year 2020–2021. Moreover, as a secondary objective, feasibility of a by-monthly testing strategy is also assessed. Materials and methods Study design and population In this study, were included 2,007 students and 520 school staff (teaching and non-teaching staff, such as extracurricular education instructors and administrative personal) who previ- ously signed the informed consent, from seven schools all over Catalonia. Although they are an opportunistic sample, epidemiological, and sociodemographic characteristics of the area, as well as type of school (public, private or chartered) were considered to assure heterogeneity. During the study period, we used two methodological approaches: a cross-sectional study to estimate SARS-CoV-2 prevalence, and a longitudinal study to calculate the COVID-19 inci- dence and evaluate the feasibility of the twice-monthly testing strategy. The cross-sectional component included students aged 3–19 from preschool (3–5 year- old), elementary school (7–10 year-old), middle school (12–15 year-old), high school (16–17 year-old) and vocational training (17–19 year-old) and school staff. For the longitudinal component, were included in a cohort of 1,424 participants, 983 stu- dents over 12 years-old and 441 school staff. PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 3 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Data collection The cross-sectional component was performed between February 22 and March 22, 2021, and the longitudinal component, were proceeded through 4 data collection rounds, between 6–19 April 2021, 20 April 2021 and 03 May 2021, 04 May 2021 and 18 May 2021 being the last round between 19 May 2021 and 02 June 2021 (Fig 1). We collected nominal data from an online questionnaire but, when necessary, a paper form was used. Questions about demographic and economic characteristics, health status, knowledge, perceptions and behaviors related to COVID-19, control measures, pandemic impacts, previous SARS-CoV-2 infection, symptoms and contacts pattern as health Fig 1. Flowchart of population at cross-sectional and longitudinal prospective components of the study. https://doi.org/10.1371/journal.pone.0277764.g001 PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 4 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year indicators were included according to COSMO questionnaire [25]. In each longitudinal round, participants filled in an additional online epidemiological survey with information related to SARS-CoV-2 infection, suspected symptoms, exposures, and vaccine status during the previous 15 days. Three different questionnaire models were designed one for school staff (questionnaire A); one for students under 16 years, which were answered by parents/guardian (questionnaire B) and one for students over 16 years (questionnaire C). Secondary data about vaccine coverage and socioeconomic level was provided by the Agèn- cia de Qualitat I Avaluació Sanitàries de Catalunya (AquAS) through the Primary Care Ser- vices Information System (SISAP) and Data Analytics Program for Health Research and Innovation (PADRIS), which collect programmatic data from different sources. The variable socioeconomic level was based on the sanitary regions and was used to categorize the place of residences in tertiles (high, medium and low). Biological samples were collected from all participants. A finger prick blood was collected to perform a rapid serological anti-SARS-CoV-2 IgM/IgG test to estimate the initial and final seroprevalence in February and June 2021 respectively. Saliva and nasal swab were col- lected twice a month to investigate the presence of SARS-CoV-2 RNA and SARS-CoV-2 antigens. All results were uploaded to the electronic health record, and, become available to the par- ticipants normally within 48 hours of the sample collection. Independent variables Factors that could have impact in outcome were referred to as independent variables, that were tested to investigate the association with the SARS-CoV-2 infection. They were catego- rized in sociodemographic, health status, contact patterns, knowledge and perceptions and, preventive measures. Each variable was coded according to the type of the question asked in the questionnaire (Table 1). Laboratory assays A RT-PCR assay (Allplex SARS-CoV-2/FluA/FluB/RSV, Werfen, Korea) and a molecular assay based on the transcription mediated amplification assay (TMA) (Procleix SARS-CoV-2, Grifols, Barcelona, Catalonia, Spain) was conducted to detect SARS-CoV-2 RNA. The nasal swab samples for detection of SARS-CoV-2 antigen were processed using the Panbio COVID- 19 Ag Rapid Test (Abbot, Chicago, IL, USA), following the manufacturer’s instructions, with a sensitivity of 93.3% (95% CI: 83.8–98.2%) and specificity of 99.4% (95% CI: 97.0–100%). For the anti-SARS-CoV-2 IgM/IgG test we used a rapid SARS-CoV-2 serological test (COVID-19 IgG/IgM Rapid Test Kit, Lambra, Madrid, Spain), following the manufacturer’s instructions, with sensitivities of 97.2% (IgG) and 87.9% (IgM), and specificities of 100% for both immuno- globulins, but following recommendations to the Public Health Protocol for COVID-19 [22], because of the low specificity of IgM antibodies and several reported cross-reactions with other non-specifics proteins, only IgG antibodies were used to assess the prevalence ofSARS- CoV-2. The samples that SARS-CoV-2 were detected were stored in the sample collection C.0001145 located at the Vall d’Hebron Hospital Universitari (Barcelona, Spain) in the sample collection registered at the Instituto de Salud Carlos III register, Madrid, Spain.Saliva samples with positive SARS-CoV-2 results were frozen and stored at the IGTP-HUGTiP Biobank, Badalona, Catalonia, Spain, and maintained for two years. PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 5 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Table 1. Independent variables included in the study, CSSNC Catalonia, Spain 2021. Sociodemographic Sex Male / Female Age Years [IQR] Parents’ occupation Dichotomized: Employed / unemployed, retired and lay-of Parents’ or school staff level of completed Higher studies or university / Secondary school / Primary school or studies None House size (in meters) Dichotomized: >70m2 / <70m2 Economic situation Changed to worse / improved Socioeconomical status Low / middle / high Parents or Staff changed employment status yes / no during the pandemic Contact pattern Contact with suspected or confirmed COVID- yes / no 19 cases (Unspecific) Place of contact with suspected or confirmed Home / school / other COVID-19 cases Living with a healthcare professional yes / no Main mode of transport On foot / bicycle / own motorcycle or car / public transport / school bus Avoiding contact in crowded spaces Likert 5-point scale dichotomized: No (never, almost never or sometimes) Yes (most of the time or always) Health Status Self-reported health status Likert 7-point scale dichotomized: Low (1–4) / High (5–7) Underlying medical conditions yes / no Specific underlying medical conditions Hypertension / Asthma / Obesity / Diabetes mellitus / Chronic heart disease Knowledge and perceptions Perceived knowledge Likert 5-point scale dichotomized: No (never, almost never or sometimes) Yes (most of the time or always) Perception of risk Likert 7-point scale dichotomized: Low (1–4) / High (5–7) Compliance of preventive measures in the last 7 days Washing hands yes / no Avoiding close contact with someone who is yes / no infected or at risk Avoiding crowded spaces or crowds yes / no Avoiding closed or indoor spaces yes / no Ventilating closed spaces whenever possible yes / no Wearing a mask yes / no Self-isolation yes / no Frequently disinfecting used objects yes / no Avoiding public transportation yes / no Use of hand sanitizing gels yes / no Avoiding touching the face, eyes, mouth with yes / no unwashed hands Staying at home if I have a cold or other illness yes / no Avoiding trips abroad yes / no Covering the mouth with the elbow when yes / no coughing or sneezing https://doi.org/10.1371/journal.pone.0277764.t001 PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 6 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Outcomes and case definitions Our first outcome was previous exposure to SARS-CoV-2 virus. The case definition for posi- tive, was any individual with a positive SARS-CoV-2 IgG antibodies detected by rapid test. The second outcome was active SARS-CoV-2 infections. The case definition for positive was any individual, symptomatic or asymptomatic with a positive RT-PCR or RAT detected by the project team or detected and confirmed by RT-PCR or RAT performed by primary health care, during the follow-up period. We decided to include these self-reported docu- mented infections because students and school staff with positive results started the isolation protocol and were no longer tested at school. Data analysis We calculated crude and adjusted prevalence for students of 2–20 years of age on the census in Catalan schools, adjusting for age and sex. For school staff, we only adjusted for sex and then by sensitivity and specificity of the tests. The differences between initial and final seropreva- lence were only calculated for students from first grade of middle school or older and staff, using the McNemar test. Descriptive analysis was performed, and the data were provided globally and stratified by educational stage when possible and presented considering sociodemographic and socioeco- nomic indicators; contact pattern; knowledge, behavior and perceptions of COVID-19 and health status. Frequency, measures of central tendency (mean and median) and dispersion (standard deviation and IQR) were calculated. Univariate analysis was performed to investigate the association between independent vari- ables and outcome, variables with p-value <0.050 were considered statistically significant. The prevalence ratio with 95% confidence interval (CI) were calculated using an adjusted Poisson model with robust error and adjustment for age, sex and school. The combined qualitative var- iables were compared using the McNemar test. For the multivariate analysis, we proceeded an initial correlation graph (polychoric correla- tion) was constructed for each category of variables and of the pairs that had a correlation coef- ficient greater than 0.8 (in absolute value), only one of the variables was used for the analysis. To fit a multivariate model, we performed the analysis with stepforward and stepwise regres- sion, ie. starting with the model with all variables, then removing them one by one and subse- quently starting with an empty model adding the variables one by one. We used the same type of regression as for univariate models (GLM Poisson with robust errors). We selected variables by significance, R2 and AIC, and both models (stepforward and stepwise regression) gave us the same model with R2 0.184547 and AIC 1261.318, compatible with the set of behavioral var- iables included in the model. We checked the Goodness of fit for the multivariate model and observed that no overdispersion was found in the Poisson model. To calculate the incidence rate for the at-risk population, we used the number of partici- pants with a positive result in RT-PCR or RAT divided by person time at risk. For at risk of infection we excluded individuals with RT-PCR or RAT positive result in the previous 60 days before started the cohort. The denominator was defined as the sum of the time at risk of the 1,366 participants sampled during the cohort (950 students and 416 employees). Time at risk was defined for each participant, as the difference of time between the moment that they entered the study and the endpoint when they tested positive or, if they did not obtain any pos- itive result, the last round that they were tested. The result was presented per 100 person- month. In the longitudinal component, a univariate analysis was performed included the same vari- ables that had been tested in the cross-sectional component, adding the information collected PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 7 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year during the follow-up endpoints. These data were assessed using independent log binomial mixed models to calculate the Relative Risk (RR) with participant’s identifier as random effect and adjusting for age, sex and school. Due the low number of positives, a multivariate model for the longitudinal component was not proceed. For the univariate analysis, a GLMM log binomial model with ID as random intercept was used to estimate the PR and RR of each variable of interest, by means of an adjusted measure and avoiding the confusion of the variables age, sex and school. Two composite indicators were created to measure the participant’s knowledge of COVID- 19: “perceived knowledge” and “factual knowledge” and another indicator to measure the “risk perception”. The “perceived knowledge” and “risk perception” were measured using a Likert 7-point scale, 1–4 scores were considered as low level of knowledge or low level of risk percep- tion and 5–7 as high level. And “factual knowledge” was measured by a binary score composed of three aspects: people at risk, symptoms and means of transmission. The answers were counted, and we classified as high level of factual knowledge when more than 50% of answers were correct and 50% or less as low. All analyses were carried out with R (version 4.1.0). Confidence intervals for incidence were obtained using the ‘epi.conf’ function from ‘epiR’ package The number of samples that should be tested to find a positive was calculated using Ene 3.0. Ethics statement The Foundation University Institute for Research in Primary Health Care Jordi Gol i Gurina (IDIAPJGol) approved the study on 17 December 2020 (code 20/192-PCV). Informed consent was obtained from school staff, parents for those children under 16 and alumni aged 16 or older. Participants were free to decline/withdraw consent at any time without providing a rea- son and without being subject to any resulting detriment. Results For participants students, except for the preschool, most participants were female (55%) over- all. Regarding socioeconomic variables, 821 (41.7%) students’ fathers and 1,043 mothers (52.3%) have high levels of study or university qualifications, 1,705 (86.7%) of the students’ fathers and 1613 (81.0%) of the students’ mothers were employed in the study period (Table 2). For school staff, mean [IQR] age was 43 [IQR 33–51], 410 (78.8%) were female, 418 (80.5%) had a high level of studies or university qualifications and 292 (56.2%) used their own car as the main means of transport. About 20% (104) had a comorbidity, the most common was asthma 23 (22.1%) and hypertension 21 (20.2%) (Table 3). Regarding epidemiological data for SARS-CoV-2 exposure risk, for students, the most com- mon place of contact with suspected or confirmed case was at the school, being 65 (65.0%) in preschool group for school staff, 122 (63.2%) reported having contact with a suspected or con- firmed case at the school (Table 4). Seroprevalence, univariate and multivariate analysis in the cross-sectional component The baseline seroprevalence of SARS-CoV-2 IgG for students and school staff was, respec- tively, 14.8% (95% CI: 13.1–16.5) and 22% (95% CI: 18.3–25.8); adjusted for sensitivity and specificity it was 15.2% (95% CI: 13.5–17) and 22.6% (95% CI: 19–26.7) and the weighted sero- prevalence was 14.5% (95% CI: 12–17.1) and 22.0% (95% CI: 21.2–22.8). The seroprevalence of SARS-CoV-2 IgG at the end of the longitudinal component for students over 12 years-old PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 8 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Table 2. General characteristics of the students participating in the sentinel school project, Catalonia, Spain. February-March 2021. Variables Preschool Elementary school Middle school High school Vocational Training N = 223 N = 752 N = 599 N = 316 N = 117 Sociodemographic and socioeconomic indicators n % n % n % n % n % Sex Female 108 48.4% 387 51.5% 306 51.1% 211 66.8% 92 78.6% Male 115 51.6% 365 48.5% 293 48.9% 105 33.2% 25 21.4% Age (years) 4.00 9.00 [7.00;10.0] 13.0 [12.0;15.0] 17.0 [16.0;17.0] 18.0 [17.0;19.0] [3.00;5.00] Level of completed studies (father) Without formal education or incomplete primary Education 10 4.6% 36 5,00% 22 3.7% 4 1.3% 14 12,00% Primary school certificate 38 17.4% 140 19.2% 135 22.9% 44 13.9% 40 34.2% Secondary school certificate 63 28.8% 208 28.6% 159 26.9% 93 29.4% 33 28.2% Higher studies or University 105 47.9% 329 45.2% 224 38,00% 154 48.7% 9 7.7% Don’t know 0 0.0% 2 0.3% 33 5.6% 18 5.7% 16 13.7% Not applicable 3 1.4% 13 1.8% 17 2.9% 3 1,00% 5 4.3% Level of completed studies (mother) Without formal education or incomplete primary Education 11 5.0% 43 5.8% 27 4.6% 4 1.3% 12 10.3% Primary school certificate 22 10.0% 76 10.2% 91 15.3% 28 8.9% 40 34.2% Secondary school certificate 42 19.1% 193 25.8% 168 28.3% 88 27.8% 34 29.1% Higher studies or University 144 65.5% 427 57.2% 275 46.4% 182 57.6% 15 12.8% Don’t know 0 0.0% 4 0.5% 26 4.4% 12 3.8% 14 12,00% Not applicable 1 0.5% 4 0.5% 6 1,00% 2 0.6% 2 1.7% Occupation (father) Not applicable 8 3.7% 27 3.7% 39 6.6% 7 2.2% 12 10.3% Others 2 0.9% 17 2.3% 8 1.4% 1 0.3% 3 2.6% Retired 0 0.0% 5 0.7% 10 1.7% 7 2.2% 8 6.8% Household keeper 2 0.9% 1 0.1% 0 0.0% 0 0.0% 0 0.0% Unemployed 5 2.3% 17 2.3% 38 6.5% 9 2.9% 4 3.4% Sick leave 3 1.4% 7 1.0% 9 1.5% 6 1.9% 6 5.1% Employed 198 90.8% 652 89.8% 485 82.3% 286 90.5% 84 71.8% Occupation (mother) Not applicable 5 2.3% 33 4.4% 23 3.9% 7 2.2% 3 2.6% Others 4 1.8% 12 1.6% 8 1.4% 1 0.3% 3 2.6% Retired 1 0.5% 0 0.0% 4 0.7% 1 0.3% 2 1.7% Household keeper 19 8.7% 42 5.6% 56 9.4% 15 4.8% 14 12.0% Unemployed 11 5.1% 29 3.9% 37 6.2% 6 1.9% 7 6.0% Sick leave 3 1.4% 12 1.6% 7 1.2% 5 1.6% 8 6.8% Employed 175 80.3% 618 82.8% 459 77.3% 281 88.9% 80 68.4% House size (m2) <50 m2 1 0.5% 8 1.1% 12 2.0% 4 1.3% 0 0.0% 51–70 m2 22 10.0% 61 8.1% 57 9.6% 13 4.1% 10 8.6% 71–90 m2 85 38.5% 235 31.4% 131 22.0% 70 22.2% 28 23.9% 91–110 m2 48 21.7% 190 25.4% 132 22.2% 67 21.2% 17 14.5% 111–130 m2 35 15.8% 111 14.8% 93 15.6% 54 17.1% 15 12.8% >130 m2 28 12.7% 142 19.0% 135 22.7% 76 24.1% 19 16.2% Don’t know 2 0.9% 4 0.5% 36 6.1% 49 15.5% 34 29.1% Underlying medical conditions No 205 93.2% 698 93.4% 538 90.1% 289 91.5% 97 82.9% (Continued ) PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 9 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Table 2. (Continued) Variables Preschool Elementary school Middle school High school Vocational Training N = 223 N = 752 N = 599 N = 316 N = 117 Sociodemographic and socioeconomic indicators n % n % n % n % n % Yes 15 6.8% 49 6.6% 59 9.9% 27 8.5% 20 17.1% Specific underlying medical conditions (n = 274) Hypertension 0 0.0% 0 0.0% 0 0.0% 1 3.7% 0 0.0% Asthma 5 33.3% 12 25.5% 31 52.5% 8 29.6% 9 45.0% Obesity 0 0.0% 0 0.0% 1 1.7% 0 0.0% 0 0.0% Diabetes mellitus 0 0.0% 1 2.1% 3 5.1% 1 3.7% 1 5.0% Chronic heart disease 2 13.3% 3 6.38% 1 1.7% 1 3.7% 0 0.0% https://doi.org/10.1371/journal.pone.0277764.t002 Table 3. General characteristics of the school staff participating in the sentinel school project, Catalonia, Spain. February-March 2021. Variables School staff N = 520 Sociodemographic and socioeconomic indicators n % Sex Female 410 78.8% Male 110 21.2% Age (years) 43.0 [33.0;51.0] Level of completed studies (school staff) Without formal education or incomplete primary Education 4 0.8% Primary school certificate 28 5.4% Secondary school certificate 69 13.3% Higher studies or University 418 80.5% House size (m2) <50 m2 15 2.9% 51–70 m2 71 13.7% 71–90 m2 143 27.5% 91–110 m2 112 21.5% 111–130 m2 72 13.8% >130 m2 91 17.5% Don’t know 16 3.1% Health status Underlying medical conditions No 416 80.0% Yes 104 20.0% Specific underlying medical conditions (n = 274) Hypertension 21 20.2% Asthma 23 22.1% Obesity 8 7.7% Diabetes mellitus 4 3.9% Chronic heart disease 2 1.9% https://doi.org/10.1371/journal.pone.0277764.t003 PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 10 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Table 4. Contact pattern of students and school staff participating in the sentinel school project, Catalonia, Spain. February-March 2021. Variables Preschool Elementary Middle school High school Vocational School staff school Training N = 223 N = 752 N = 599 N = 316 N = 117 N = 520 Lives with healthcare professional No 189 85.1% 656 87.6% 538 90.6% 274 86.7% 104 88.9% 442 92.9% Yes 33 14.9% 93 12.4% 56 9.4% 42 13.3% 13 11.1% 34 7.1% Main mode of transport On foot 152 68.2% 495 65.8% 378 63.3% 136 43.0% 22 18.8% 152 29.2% Scooter/bicycle 30 13.5% 30 4.0% 29 4.9% 33 10.4% 1 0.9% 36 6.9% Motorcycle/own car 81 36.3% 284 37.8% 173 29.0% 119 37.7% 71 60.7% 292 56.2% Public transport 21 9.4% 89 11.8% 106 17.8% 103 32.6% 39 33.3% 97 18.7% School bus 1 0.5% 5 0.7% 3 0.5% 1 0.3% 1 0.9% 0 0.0% Contact with suspected or confirmed COVID-19 cases Don’t know 72 32.6% 320 43.0% 155 26.1% 54 17.1% 22 18.8% 205 39.5% No 48 21.7% 174 23.4% 168 28.2% 100 31.6% 24 20.5% 121 23.3% Yes, with both confirmed and suspected cases 4 1.8% 22 3.0% 26 4.4% 23 7.3% 12 10.3% 40 7.7% Yes, with suspected cases 5 2.3% 13 1.8% 16 2.7% 13 4.1% 4 3.4% 14 2.7% Yes, with confirmed cases 92 41.6% 215 28.9% 230 38.7% 126 39.9% 55 47.0% 139 26.8% Place of contact At home 42 42.0% 122 49.6% 115 42.3% 72 44.4% 23 32.4% 58 30.1% At school 65 65.0% 137 55.7% 165 60.7% 82 50.6% 41 57.7% 122 63.2% Leisure activities 2 2.0% 25 10.2% 53 19.5% 54 33.3% 17 23.9% 26 13.5% Don’t know 0 0.0% 3 1.2% 0 0.0% 8 4.9% 4 5.6% 0 0.0% Others 5 5.0% 9 3.7% 17 6.3% 11 6.8% 16 22.5% 23 11.9% https://doi.org/10.1371/journal.pone.0277764.t004 and staff was 18.4% (95% CI: 15.6–21.1) and 42.6% (95% CI: 37.7–47.5), adjusted for sensitivity and specificity it was 18.9% (95% CI: 16.2–21.9) and 43.8% (95% CI: 38.8–49.0) and the weighted seroprevalence was 19.7% (95% CI: 13.3–26.1) and 42.5% (95% CI: 0–92.4). The prevalence of active SARS-CoV-2 infection confirmed by RT-PCR or RAT at baseline was 0.7% (95% CI: 0.3–1.0) in students and 1.1% (95% CI: 0.1–2.0) in school staff. Weighted preva- lence was 1% (95% CI: 0–2.1) and 1.1% (95% CI: 1–1.1). Self-reported documented infection of SARS-CoV-2 between February 2020 and March 2021 was 8.5% (95% CI: 7.3–9.8) for stu- dents and 7.1% (95% CI: 4.9–9.3) for staff. For students, when weighted by sex and age it was 8.2% (95% CI: 4.3–12.2) and for staff when weighted by sex it was 7.1% (95% CI: 5.9–8.4) (Table 5). Among those participants who had two serological tests at baseline and at the end of the longitudinal component (round four), there was a significant increase in prevalence (p<0.001). The main differences were in the staff group (p-value <0,001), although there was also a no significant increase among students in vocational studies (Fig 2). The variables included in the univariate analysis, for students and staff were presented into sociodemographic, health and behaviors and contact patterns categories. Indiscriminate changes in the employment situation (PR 1.43, CI 1.07–1.91) and improved the economic situa- tion (PR 2.66 CI 1.18–6.00) regarding parents and school staff were positively associated with having been infected. The variable higher perceived knowledge was positively associated with the infection (PR 1.68 CI 1.05–2.68). The public health measure, avoiding crowded spaces was negatively associated with the infection (PR 0.65 CI 0.45–0.93) and a higher risk perception was positively associated (PR 1.49 CI 1.14–1.93) (Table 6). PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 11 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Table 5. Summary of crude and adjusted seroprevalence and active infections during the first trimester of 2021 in sentinel school project, Catalonia, Spain. Indicator Students n = 2007 Staff n = 520 % (95%CI) % (95%CI) Seroprevalence Initial crude seroprevalence of SARS-CoV-2 IgG 14.8 (13.1–16.5) 22 (18.3–25.8) Adjusted initial seroprevalence for sensitivity and specificity 1 15.2 (13.5–17) 22.6 (19–26.7) Weighted initial seroprevalence of SARS-CoV-2 IgG 2 14.5 (12–17.1) 22.0 (21.2–22.8) Final crude seroprevalence of SARS-CoV-2 IgG (n = 1153) 18.4 (15.6–21.1) 42.6 (37.7–47.5) Adjusted final seroprevalence for sensitivity and specificity 1 (n = 1153) 18.9 (16.2–21.9) 43.8 (38.8–49.0) 2 Weighted final seroprevalence of SARS-CoV-2 IgG (n = 1153) 19.7 (13.3–26.1) 42.5 (0–92.4) Active infection Prevalence of active SARS-CoV-2 infection at baseline 3 0.7 (0.3–1.0) 1.1 (0.1–2.0) Weighted prevalence of active SARS-CoV-2 infection at baseline 2, 3 1 (0–2.1) 1.1 (1–1.1) 1 .COVID-19 IgG/IgM Rapid Test Kit, Lambra with sensitivities of 97,2% (IgG) and 87,9% (IgM), and specificities of 100% 2 . Weighted according to 2020 student censused in Catalonia age 2–20 years by sex and age and school staff by sex. Estimated prevalence of the biological samples collected by the project team or the applied questionnaire 3 . Biological samples collected during baseline, TMA with nasal swab performed by project team between Feb and Mar 2021 Abbreviations: IgG: immunoglobulin G; 95% CI: 95% Confidence interval https://doi.org/10.1371/journal.pone.0277764.t005 Fig 2. Adjusted seroprevalence of antibodies IgG anti-SARS-CoV-2 in longitudinal population by educational stages and school staff at the baseline (February-March 2021) and the last follow-up (May-June 2021). ��� p <0.001 using McNemar test. https://doi.org/10.1371/journal.pone.0277764.g002 PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 12 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Table 6. Summary and univariate analysis between SARS-CoV-2 infection and sociodemographic, health and behavioral indicators of students and staff from senti- nel schools. Catalonia, February to March 2021. Variables Students Staff Total Univariate analysis Positive Negative Positive Negative Positive Negative Prevalence ratio p-value N = 180 N = 1827 N = 41 N = 479 N = 221 N = 2306 n % n % n % n % n % n % (PR) Sociodemographic Sex - - Female 101 56.1 1003 54.9 32 78.0 378 78.9 133 60.2 1381 59.9 - - Male 79 43.9 824 45.1 9 22.0 101 21.1 88 39.8 925 40.1 - - Age (years) 15.0 12.0 45.0 43.0 15.0 13.0 - - [11.0;16.0] [8.00;15.0] [32.0;50.0] [33.5;51.0] [12.0;18.0] [9.00;17.0] Changed employment status during the pandemic No 128 73.1 1418 79.7 34 82.9 392 82.7 162 75.0 1810 80.3 ref Yes 47 26.9 362 20.3 7 17.1 82 17.3 54 25.0 444 19.7 1.43 (1.07–1.91) 0.015 Economic situation Changed to worse 40 88.9 407 96.4 3 60.0 69 82.1 43 86.0 476 94.1 ref Improved 5 11.1 15 3.6 2 40.0 15 17.9 7 14.0 30 5.9 2.66 (1.18–6.00) 0.019 Socioeconomical status Low 77 42.8 837 46.5 23 56.1 239 51.1 100 45.2 1076 47.4 ref Middle 43 23.9 542 30.1 13 31.7 132 28.2 56 25.3 674 29.7 0.74 (0.52–1.06) 0.098 High 60 33.3 422 23.4 5 12.2 97 20.7 65 29.4 519 22.9 1.12 (0.76–1.65) 0.582 Knowledge and perceptions Perceived knowledge �� Low 13 24.1 150 37.8 7 17.1 135 28.2 20 21.1 285 32.5 ref High 41 75.9 247 62.2 34 82.9 344 71.8 75 78.9 591 67.5 1.68 (1.05–2.68) 0.030 Risk perception�� Low 120 66.7 1435 79.2 17 41.5 228 47.6 137 62.0 1663 72.6 ref High 60 33.3 377 20.8 24 58.5 251 52.4 84 38.0 628 27.4 1.49 (1.14–1.93) 0.003 � Likert scale. No: never, almost never or sometimes; Yes: most of the time or always �� Likert scale. Low: 1–4; High: 5–7 ��� GLM adjusted Poisson model with robust error and adjustment for age, sex and school https://doi.org/10.1371/journal.pone.0277764.t006 In the case of contact patterns, having unspecific contact with suspected or confirmed COVID-19 case, (PR 2.76; CI 1.94–3.93) or contact at home (PR 2.17; CI 1.62–2.91) were posi- tively associated with the infection, in contrast, contact at school, had a strongly negative asso- ciation (PR 0.60 CI 0.45–0.80). Living with a health professional was not associated with infection (p-value = 0.262). (Table 7). In the multivariate analysis, the same significance and trend among socioeconomic, health measures and contact patterns variables were observed (Table 8). Incidence and univariate analysis at the longitudinal component During the longitudinal component of the study, 45 new infections occurred (34 students and 11 staff), 11 of them identified by RT-PCR performed by the project team and 34 self-reported in by the participants. It is interesting to note that out of 11 RT-PCR positives identified in the study, only 1 (9%) was also detected by RAT. The overall incidence was 2.73 (95% CI 1.991, 3.653) per 100 person-month, that is 2.887 (95% CI 1.999, 4.034) and 2.337 (95% CI 1.167, 4.182) per 100 person-month for students and staff, respectively. PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 13 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Table 7. Univariate analysis between SARS-CoV-2 infection and contact patterns of students and staff from sentinel schools. Catalonia, Spain, February-March 2021. Variables Students Staff Total Univariate analysis Positive Negative Positive Negative Positive Negative Prevalence ratio p-value N = 180 N = 1827 N = 41 N = 479 N = 221 N = 2306 n % n % n % n % n % n % Lives with healthcare professional No 163 91.1 1598 87.9 35 94.6 407 92.7 198 91.7 2005 88.8 ref Yes 16 8.9 221 12.1 2 5.4 32 7.3 18 8.3 253 11.2 0.77 (0.49–1.22) 0.262 Unspecific contact with suspected cases of COVID-19 Don’t know 24 13.4 599 33.0 13 31.7 192 40.2 37 16.8 791 34.5 ref No 21 11.7 493 27.2 4 9.8 117 24.5 25 11.4 610 26.6 0.79 (0.49–1.29) 0.349 Yes 134 74.9 722 39.8 24 58.5 169 35.4 158 71.8 891 38.9 2.76 (1.94–3.93) <0.001 Contact with suspected cases of COVID-19 at home No 52 38.8 425 59.3 12 50.0 123 72.8 64 40.5 548 61.9 ref Yes 82 61.2 292 40.7 12 50.0 46 27.2 94 59.5 338 38.1 2.17 (1.62–2.91) <0.001 Contact with suspected cases of COVID-19 at school No 66 49.3 295 41.1 15 62.5 56 33.1 81 51.3 351 39.6 ref Yes 68 50.7 422 58.9 9 37.5 113 66.9 77 48.7 535 60.4 0.60 (0.45–0.80) <0.001 Avoiding contact in crowded spaces� No 26 14.4 170 9.5 4 10.0 23 4.9 30 13.6 193 8.5 Yes 154 85.6 1625 90.5 36 90.0 452 95.2 190 86.4 2077 91.5 0.65 (0.45–0.93) 0.019 Self-reported health status�� Low 8 4.4 27 1.5 2 4.9 22 4.6 10 4.5 49 2.1 ref High 172 95.6 1794 98.5 39 95.1 457 95.4 211 95.5 2251 97.9 0.55 (0.30–1.01) 0.053 � Likert scale. No: never, almost never or sometimes; Yes: most of the time or always �� Likert scale. Low: 1–4; High: 5–7 ��� GLM adjusted Poisson model with robust error and adjustment for age, sex and school https://doi.org/10.1371/journal.pone.0277764.t007 The variables included in the univariate analysis were also categorized into sociodemo- graphic and socioeconomic indicators, health status, and preventive compliance. There was a protective behavior associated with socioeconomic status, when comparing the highest level (high) in reference to the first tercile (low) (RR 0.25, 95% CI 0.06–0.96) for the COVID-19 infection (Table 9). Contact with a suspected or confirmed case of COVID-19, was a risk factor for infection (RR 6.44, 95% CI 3.15–13.19). When the contact occurred at home, the risk (RR 12.42, 95% CI Table 8. Multivariate analysis between SARS-CoV-2 infection, sociodemographic and health indicators, by students and staff from sentinel schools. Catalonia, Feb- ruary to March 2021. Variables Estimate CI_low CI_upp p-value Contact pattern 2.205 1.876 2.591 <0.001 Perceived gravity if infected with coronavirus 1.328 1.011 1.744 0.041 Be employed 1.348 1.009 1.801 0.043 Compliance of sanitary measures 0.547 0.372 0.805 0.002 Self-reported health status 0.432 0.221 0.845 0.014 Use of transport public or school bus 0.522 0.327 0.836 0.007 Parents educational level (higher) 0.726 0.554 0.953 0.021 https://doi.org/10.1371/journal.pone.0277764.t008 PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 14 / 23 Table 9. Longitudinal component results by follow-up endpoint and relative risk adjusted by sex and age, of risk factors for SARS-CoV-2 infection in participants of the longitudinal compo- nent. Sentinel schools project, Catalonia, Spain, April to June 2021. Variables Round 1 Round 2 Round 3 Round 4 Univariate analysis Positive N = 10 Negative N = 1250 Positive N = 14 Negative N = 1182 Positive N = 6 Negative N = 1118 Positive N = 15 Negative N = 1127 RR (95% CI) p-value n % n % n % n % n % n % n % n % PLOS ONE Socioeconomic Sex Female 4 40.0 815 65.2 5 35.7 761 64.4 5 83.3 709 63.4 10 66.7 722 64.1 - - Male 6 60.0 435 34.8 9 64.3 421 35.6 1 16.7 409 36.6 5 33.3 405 35.9 - - Age—years [IQR] 15.5 [14.2;16.8] 16.0 [14.0;31.0] 15.0 [13.0;16.0] 16.0 [14.0;32.0] 14.5 16.0 [13.0;32.0] 16.0 [14.0;34.5] 16.0 [14.0;34.0] - - [12.5;34.5] Socioeconomic level Low 5 50.0 657 53.2 10 71.4 629 53.9 2 33.3 553 50.1 8 53.3 587 52.9 ref Middle 4 40.0 351 28.4 3 21.4 319 27.4 4 66.7 336 30.4 6 40.0 324 29.2 0.91 (0.43–1.93) 0.814 High 1 10.0 226 18.3 1 7.14 218 18.7 0 0.0 215 19.5 1 6.7 199 17.9 0.25 (0.06–0.96) 0.044 Contact pattern� Had contact Don’t know 3 33.3 607 59.4 4 36.4 452 60.6 2 33.3 247 60.4 8 66.7 341 69.0 ref No 4 44.4 347 34.0 2 18.2 247 33.1 0 0.0 132 32.3 2 16.7 134 27.1 0.74 (0.32–1.75) 0.497 Yes 2 22.2 68 6.7 5 45.5 47 6.3 4 66.7 30 7.33 2 16.7 19 3.9 6.44 (3.15–13.19) <0.001 Contact at home No 8 88.9 1001 97.9 8 72.7 742 99.5 4 66.7 400 97.8 11 91.7 492 99.6 ref Yes 1 11.1 21 2.1 3 27.3 4 0.5 2 33.3 9 2.2 1 8.3 2 0.40 12.42 (5.81–26.52) <0.001 Contact at school PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 No 8 88.9 985 96.4 10 90.9 710 95.2 4 66.7 396 96.8 11 91.7 479 97.0 ref Yes 1 11.1 37 3.6 1 9.1 36 4.8 2 33.3 13 3.2 1 8.3 15 3.0 3.73 (1.49–9.38) 0.005 Unspecific contact No 8 88.9 990 96.9 9 81.8 729 97.7 4 66.7 400 97.8 12 100 490 99.2 ref Yes 1 11.1 32 3.1 2 18.2 17 2.3 2 33.3 9 2.2 0 0.0 4 0.8 5.28 (2.1–13.27) <0.001 Health status Chronic diseases No 6 60.0 1083 86.8 13 92.9 1023 86.7 3 50.0 965 86.5 12 80.0 979 87.0 ref Yes 4 40.0 165 13.2 1 7.1 157 13.3 3 50.0 151 13.5 3 20.0 146 13.0 2.18 (1.1–4.33) 0.026 Preventive measures� � Wearing a mask No 0 0.0 8 0.8 1 10.0 5 0.7 1 16.7 1 0.3 0 0.0 1 0.2 ref Yes 9 100 1012 99.2 9 90.0 741 99.3 5 83.3 404 99.8 11 100 490 99.8 0.14 (0.04–0.53) 0.004 Avoid contact� No 2 22.2 43 4.5 1 10.0 39 5.6 1 16.7 20 5.3 1 9.1 25 5.5 ref Yes 7 77.8 921 95.5 9 90.0 656 94.4 5 83.3 361 94.8 10 90.9 429 94.5 0.38 (0.15–0.97) 0.043 � With people suspect, confirmed or at risk for COVID-19 �� In the last 7 days ��� Calculated using an independent log binomial mixed model https://doi.org/10.1371/journal.pone.0277764.t009 15 / 23 Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year 5.81–26.52) was higher than compared to school (RR 3.73, 95% CI 1.49–9.38) and other non- specific locations (RR 5.28, 95% CI 2.1–13.27). We tested several sanitary measures that had been carried out in the last seven days before the survey and only avoiding close contact with someone who is infected or at risk (RR 0.38, 95% CI 0.15–0.97) and a wearing mask (RR 0.14, 95% CI 0.04–0.53) were associated, this is compat- ible with the also significant result of the variable contact with a suspected or confirmed cases of COVID-19. We tested the feasibility of a twice a month RT-PCR testing strategy. Considering a preva- lence of 0.07% and accuracy of 0.05) 1,258 participants should be tested to find one positive. Discussion In Switzerland, the “Ciao Corona” study, conducted in June/July 2020, October/November 2020, and March/April 2021 with 2,585 children, found 2.8% (95%CI 1.6–4.1%) SARS-CoV-2 IgG, IgM and IgA seroprevalence [26, 27]. In Germany, a study conducted during May and June 2020 founded 0.6% seroprevalence for students and school staff and 0.7% at the follow up, in September/October 2020 [28]. A population study carried out with children under 18 years of age in Catalonia found a lower seroprevalence than what we found in our study, of 4.4% between March and April 2020. This difference may reflect the difficulty of diagnosing asymptomatic youngers especially during the initial period of the pandemic, emphasizing the importance of active surveillance of school sentinel populations, for the timely detection of respiratory viruses [29]. As expected, in our study there was a significant increase in SARS-CoV-2 seroprevalence, in the school staff group, which can be explained by the increase of vaccination coverage. According to PADRIS data the vaccine coverage in school staff went from 78% and 0.6% in April 2021 to 84.3% and 35.6% in June 202, partly and fully vaccinated respectively. At the time of seroprevalence data collection in this study, vaccines were not approved for people under 18 years. The prevalence of active SARS-CoV-2 infections detected by the project was low consider- ing the overall prevalence and incidence from Catalonia during the same period [3, 20, 30, 31]. This suggests that public health strategies such as testing of symptomatic individuals and con- tact tracing efforts were effective at identifying an active infection at school, even the asymp- tomatic population [28]. Another study proceeded during in December 2020, in a high community transmission period in Switzerland [17] found, a positive PCR in none of the teacher and one child and Antigen positive test in 7 (1.1%) children and 2 (3.0%). Considering detected and self-reported infections in our longitudinal study we found a low incidence of COVID-19 infections, consistent with other studies that have very similar results to ours [28, 32–34]. Also, there are studies suggesting that higher community incidence, diag- nostic issues [30], demographic and economic aspects are determinants in the variation of dif- ferent rates detected, as showed in these studies [16, 35, 36]. The association between socioeconomic status and SARS-CoV-2 infection was different depending on the period of data collection. First, at the beginning of the pandemic, improved economic situation was positively associated with having been infected. This could be explained by the fact that the most affected population were those who worked and travelled than those who were respecting the lockdown measures. Then, during the follow-up we observed a new trend where a higher infection risk was associated with lower economic status. This provides important clues to understanding the COVID-19 burden in different economic and demographic contexts [37]. Population-based studies found similar results where hetero- geneity in incidence and mortality rates [32, 35, 36] were associated with socioeconomic status PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 16 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year showing the importance of planning sanitary policies oriented to the territorial characteristics and specific inequities [38, 39], such as in a follow-up study in Brazil that found a high inci- dence in children living in a slum area [16]. At baseline, contact with suspected or confirmed cases, especially at home, was positively associated with SARS-CoV-2 infection, as observed in a study that found that physical distanc- ing measures, including limited close contacts while school remained open, controlled SARS- CoV-2 transmission [40]. However, school contacts had a negative association with this out- come, showing how well-implemented sanitary protocols make the safe opening of schools possible, consistent with other studies that found an association between low transmission and, sanitary recommendations and preventive measures [6, 41]. In the longitudinal component analysis, all contact patterns were a risk factor, especially when contact was at home, consistent with previous studies that demonstrated an increased risk of infection associated with household contacts in Catalonia [15, 31, 42] and modeling studies that demonstrated an increased risk for infection in household contacts [43, 44]. Even contact tracing studies found no typical or frequent child-adult transmission [16, 45, 46] and a low contribution of children in the secondary cases [47, 48], which showed that children do not seem to be the main source of infection [11, 49]. Interestingly, contact with healthcare professionals was not associated with the infections in our study. Our hypothesis to explain that is the compliance of preventive measures at home when health care workers were exposed to risk situations, however, we need more studies to understand the role of HCW in this transmission model. Perceived knowledge was positively associated with infection, which may indicate either knowledge acquired due to a previous infection, or the large amount of lay knowledge con- sumed by the young and indeed the general population [49, 50]. Knowledge of COVID-19 and risk perception may have been due to the occurrence of a previous infection, which would not explain the occurrence of a later infection. A high level of risk perception of exposure might indicate they understood the risks they had taken. Other studies also show that risk perceptions, behaviors and compliance with sanitary measures are associated with levels of knowledge [51–53]. As with other studies [15, 17, 47] our results reinforce that the transmission by children in the school setting did not appear to make a major contribution to the spread of the virus, especially for the youngest children. This supports the decision of many countries to keep schools open while following several public health measures and safety protocols to control the transmission of the virus. Our study also reinforces the idea that the strategy based on an active sentinel surveillance for detection of acute SARS-CoV-2 infections followed by isola- tion of bubble groups seems to be more effective in scenarios with susceptible groups and rapid transmission. Approximately half of the target population agreed to participate and considering the diffi- cult circumstances schools and families were experiencing because of the pandemic; we con- sider this proportion to be quite acceptable. As a matter of fact, it is similar to other studies where 75% and 25% of students and staff participated respectively [28] or with 49% of child participation [17]. Limitations Although the overall participation rate in our study was 45.4%, it was proportionately higher among school staff (72%) than students (41%), this suggests that given the fact that higher sociodemographic heterogeneity (nationality, language, socioeconomic status) was higher among students than staff, some of these factors could have also influenced participation. PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 17 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year There were no difficulties in implementing the study and all sentinel schools gave us excellent feedback for the associated activities. Because of the sample of the schools and the participation rate, the study population may not be representative of all schools in Catalonia. Nevertheless, the heterogeneity of the included school’s information from different socioeconomical scenarios with a big enough study population. As a sentinel population approach, the objective of the CSSNC is not to extrapolate parameters, but to complement formal epidemiological surveillance systems by means of monitoring them steadily over time population studied. The data presented were gathered before the Omicron variant circulation and therefore our findings may not apply during the Spanish sixth wave or other future scenarios related to potential new variants and vaccine recommendations for children. As a cross-sectional design, association should be interpreted with caution, without attrib- uting causality. Variables such as distal characteristics must be interpreted differently from variables that can change over time such as knowledge, behavior, and contact patterns, which are influenced by the occurrence of the disease. Moreover, the acceptability, compliance, and prevention behaviors, may have been directly affected by the course of the pandemic. There were some limitations to our longitudinal analysis as the small number of acute infec- tions made impossible to apply a multivariate analysis. Also, with community public health measures occurring simultaneously with the schools’ own protocols it was difficult to evaluate these determinants separately. In addition, there was a poor distribution of confounders between groups, which can also have very different sizes, resulting in a loss of statistical power in a multivariate model. Conclusions This study offers a unique perspective on the prevalence and incidence of SARS-CoV-2 infec- tion among students and school staff in Catalonia, an important result considering the diffi- culty of detecting the virus among asymptomatic young people, as well as regarding the compliance and effectiveness of public health measures implemented in these schools in the transmission of SARS-CoV-2. The CSSNC demonstrated, for the first time in Spain, the feasibility of correlating individ- ual socio-epidemiological data and data on the prevalence and incidence of SARS-CoV-2 in the school environment, even during the difficult acute period of the pandemic. Despite the high prevalence and community incidence of SARS-CoV-2 in Catalonia during the study period, this project found a low prevalence and incidence of active infections in the school population, suggesting that the prevention methods adopted by schools, together with other strategies of health care, such as testing and contact tracing, were effective in containing trans- mission in educational settings. Monitoring of SARS-CoV-2 biological markers and their behavioral and structural deter- minants over time in sentinel schools is crucial to assess the situation of the COVID-19 pan- demic and provide relevant information to inform guidelines and policies to increase safety among students and staff in school environments. Apart from identifying multilevel transmis- sion determinants for SARS-CoV-2 among students and school staff, they may also be useful to describe the spread of other infectious diseases such as influenza and other respiratory viruses and facilitate healthier learning environments for all. Acknowledgments The authors would like to thank the Health Department and Department of Education of the Government of the Catalonia (Spain), the former Direcció General de Recerca i Innovació en PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 18 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Salut (DGRIS) and Institut Català de la Salut (ICS) that made this project possible, and all the health care professionals acting as a COVID-19 pandemic health taskforce in Catalonia. Also, we would like to specially thank the effort and dedication all the sentinel schools’ staff, students and families who participated in the project. COVID-19 Sentinel Schools Network of Catalonia (CSSNC) Principal investigators: Jordi Casabona (jcasabona@iconcologia.net) [Centre d’Estudis Epi- demiològics sobre les Infeccions de Transmissió Sexual i Sida de Catalunya (CEEISCAT)- CIBERESP], and Josep Basora (Institut Universitari d’Investigació en Atenció Primària IDIAP Jordi Gol). Project manager: Anna Bordas (CEEISCAT). Technical committee: Jordi Casabona (CEEISCAT), Jordi Sunyer (ISGlobal), Pere Soler- Palacı́n (Hospital Universitari Vall d’Hebron), Rosina Malagrida (Living lab for Health, IRSI- Caixa) as Work Package coordinators. Cinta Folch, Pol Romano, Esteve Muntada, Anna Bor- das, Fabiana Ganem, Andreu Colom-Cadena i Jordi Casabona (CEEISCAT), Mireia Gascón, Maria Subirana, Pau Majo, Jordi Sunyer (ISGlobal), Rosina Malagrida, Laia Vives (Living lab for Health, IrsiCaixa), Antonio Soriano (Hospital Universitari Vall d’Hebron), Josep Vidal (Gerència Territorial de la Catalunya Central, Institut Català de la Salut). Microbiology laboratories: Tomàs Pumarola, Andrés Antón, Cristina Andrés, Juliana Esperalba, Albert Blanco (Hospital Universitari Vall d’Hebron), Ignacio Blanco, Pere-Joan Cardona, Maria Victoria González, Gema Fernández, Cristina Esteban (Hospital Universitari Germans Trias i Pujol) Data Management and statistical analysis: Yesika Dı́az, Lucia Alonso, Jordi Aceiton, Marcos Montoro (CEEISCAT). Data Protection Officer and Technical Support: Esteve Muntada (CEEISCAT). Communication manager: Pol Romano, Cristina Sànchez (CEEISCAT). Field team: Maria Subirana, Pau Majo (ISGlobal), Jessica Fernández, Laia Vives (Living Lab for Health, IRSICaixa), Andreu Colom-Cadena, Isabel Martı́nez, Marina Herrero, Alba Garcı́a, Juan Rus, Paula Ribas, Alba Blanco (CEEISCAT). Community Pediatricians: Esperança Macià i Silvia Burgaya (CAP Manlleu), Mª Teresa Riera-Bosch, Elisabet Sola (EAP Vic Nord), Lidia Aulet, Maria Mendoza, Lidia Busquets (EAP Vic Sud), Xavier Perramon, Júlia Sebastià (EAP Eixample Dret), Ana Moreno (Cap Ripollet), Xavier Duran, Belen Pérez (EAP Can Gibert del Pla), Anna Gatell (Equip Territorial de Pedia- tria Alt Penedès), Maria Coma (Hospital Universitari Joan XXIII). Epidemiology suveillance services: Marc Olivella, Irene Barrabeig, Ignacio Parrón, Mònica Carol, Núria Bes, Pere Godoy, Sofia Minguell, Maria Rosa Sala. Department of Health: Ariadna Mas (Direcció Assistencial Atenció Primària, Institut Català de la Salut), Maria Antònia Llopis (Coordinació dels laboratoris de l’Institut Català de la Salut), Sandra Pequeño and Jacobo Mendioroz (Subdirecció general de Vigilància i Resposta a Emergències de l’Agència de Salut Pública de Catalunya), Laia Asso (Agència de Salut Pública de Catalunya) Aina Plaza, Montserrat Llavayol (Direcció Promoció de la Salut, Depar- tament de Salut), and Carmen Cabezas (Secretaria Salut Pública). Department of Education: Josep Gonzàlez-Cambray (Conseller d’Educació), Núria Mora (Secretaria de Transformació Educativa), Maria Neus Fornells (Gabinet del Conseller), Rut Ribas (Direcció general de l’alumnat). Author Contributions Conceptualization: Fabiana Ganem, Anna Bordas, Cinta Folch, Jordi Casabona. PLOS ONE | https://doi.org/10.1371/journal.pone.0277764 November 17, 2022 19 / 23 PLOS ONE Associated factors of SARS-CoV-2 infection in educational settings during the 2020-2021 academic year Data curation: Anna Bordas, Cinta Folch, Lucia Alonso, Marcos Montoro-Fernandez. Formal analysis: Fabiana Ganem, Anna Bordas, Lucia Alonso, Marcos Montoro-Fernandez. Investigation: Fabiana Ganem, Anna Bordas, Cinta Folch, Andreu Colom-Cadena, Ariadna Mas, Jacobo Mendioroz, Laia Asso, Andres Anton, Tomàs Pumarola, Maria Victoria Gon- zález, Ignacio Blanco, Pere Soler-Palacı́n, Antoni Soriano-Arandes, Jordi Casabona. Methodology: Fabiana Ganem, Anna Bordas, Cinta Folch, Lucia Alonso, Jordi Casabona. Project administration: Anna Bordas, Jordi Casabona. Resources: Ariadna Mas, Jacobo Mendioroz, Laia Asso, Andres Anton, Tomàs Pumarola, Maria Victoria González, Ignacio Blanco, Pere Soler-Palacı́n, Antoni Soriano-Arandes, Jordi Casabona. Supervision: Jordi Casabona. 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