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BackgroundMuch remains unknown regarding the evolution of SARS-CoV-2 seroprevalence and variability in seropositive children in districts, schools, and classes as only a few school-based co-hort studies exist. Vaccination of children, initiated at different times for different age groups, adds additional complexity to understand how seroprevalence developed in the school aged population. AimWe investigated the evolution of SARS-CoV-2 seroprevalence in children and its variability in districts, schools, and classes in Switzerland from June/July 2020 to November/December 2021. MethodsIn this school-based cohort study, SARS-CoV-2 antibodies were measured in primary and secondary school children from randomly selected schools in the canton of Zurich in October/November 2020, March/April 2021, and November/December 2021. Seroprevalence was estimated using Bayesian logistic regression to adjust for test sensitivity and specificity. Variability of seroprevalence between school classes was expressed as maximum minus minimum sero-prevalence in a class and summarized as median (interquartile range). Results1875 children from 287 classes in 43 schools were tested, with median age 12 (range 6-17), 51% 12+ vaccinated. Seroprevalence increased from 5.6% (95% CrI: 3.5-7.6%) to 31.1% (27.0-36.1%) in unvaccinated children, and 46.4% (42.6-50.9%) in all children (including vaccinated). Earlier in the pandemic, seropositivity rates in primary schools were similar to or slightly higher (<5%) than those in secondary schools, but by late 2021, primary schools had 12.3% (44.3%) lower seroprevalence for unvaccinated (all) subjects. Variability in seroprevalence among districts and schools increased more than twofold over time, and in classes from 11% (7-17%) to 40% (22-49%). ConclusionSeroprevalence in children increased greatly, especially in 2021 following introduction of vaccines. Variability in seroprevalence was high and increased substantially over time, suggesting complex transmission chains. Trial Registration: ClinicalTrials.gov NCT04448717

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We measured SARS-CoV-2 seroprevalence in cohorts of children, parents and school personnel in 55 randomly selected schools in the canton of Zurich, Switzerland. In June-September 2020, seroprevalence was low (4.4% to 5.8%) in all cohorts. In March-April 2021, seroprevalence in children and parents (18.1% and 20.9%) was slightly higher than in school personnel (16.9%). Childrens seroprevalence was slightly higher in classes with infected main teachers and families with one infected parent, and substantially higher in families with two infected parents.

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BackgroundPrevious studies assessing the impact of the COVID-19 pandemic on childrens and adolescents lifestyle focused mainly on the first wave in early 2020. We aimed to describe changes in adherence to recommendations for physical activity (PA), screen time (ST), and sleep duration over the first two waves of the pandemic (March-May 2020 and October 2020-January 2021) in Switzerland, and to assess the associations of these lifestyle behaviours with life satisfaction and overall health, as indicators of well-being. MethodsWe included 3168 participants aged 5 to 18 years from four Swiss cantons. Participants or their parents completed repeated questionnaires and reported on their (childs) PA, ST, sleep, life satisfaction, and overall health. We analysed lifestyle behaviours in terms of adherence to international recommendations. We used linear and logistic regression models to assess the associations of number of recommendations met and adherence patterns with well-being indicators. FindingsCompared to the pre-pandemic period, the percentage of participants meeting the recommendations for PA and ST decreased strikingly during March-May 2020, while there was a slight increase in those meeting recommendations for sleep. During October 2020-January 2021, the percentage of compliant children for PA and ST increased but remained lower than before the pandemic. Participants meeting all three recommendations were more likely to report excellent health (OR: 1{middle dot}87 [1{middle dot}15-3{middle dot}08]) and a higher life satisfaction score ({beta}: 0{middle dot}59 [0{middle dot}30-0{middle dot}88]) than participants not meeting any recommendation. Adherence to recommendations for PA and sleep, PA and ST, and sleep and ST was similarly associated with both well-being indicators. InterpretationWe show a substantial impact of the COVID-19 pandemic on childrens and adolescents lifestyle behaviours with a partial recovery over time, and an association between lifestyle and well-being. Public health policies to promote childrens and adolescents well-being should target PA, ST, and sleep simultaneously. FundingCorona Immunitas. RESEARCH IN CONTEXTO_ST_ABSEvidence before the studyC_ST_ABSWe searched PubMed for studies assessing the effects of the COVID-19 pandemic on childrens and adolescents lifestyle behaviours, published up to September 6, 2021, with no language restrictions. Of the studies found, nearly all compared lifestyle behaviours before and during the strict confinement in the first wave of the pandemic, and very few studies extended their assessment beyond June 2020. The only longitudinal study assessing lifestyle changes up to 2021 included a sample of nineteen boys. Some studies assessed the association between lifestyle behaviours and well-being after the outbreak of the pandemic, but all used a single-behaviour approach (i.e., evaluated only one lifestyle behaviour) and no study considered the combined contribution of physical activity (PA), screen time (ST), and sleep. In addition, most studies were cross-sectional and did not consider pre-pandemic lifestyle behaviours. Added value of this studyThis is the first study assessing changes in adherence to international recommendations regarding PA, ST, and sleep duration in children and adolescents during the first two waves of the COVID-19 pandemic and the joint associations of these lifestyle behaviours with well-being. We used data from 3168 children and adolescents from four different Swiss cantons. We included measurements of PA, ST, and sleep before the pandemic, during the first wave between March and May 2020, and during the second wave between October 2020 and January 2021. We also assessed life satisfaction and overall health as indicators of well-being, between January and April 2021. We showed that, compared to the pre-pandemic period, the proportion of children and adolescents following the recommendations for PA and ST decreased during the first wave (lockdown period), while there was a slight increase in those meeting recommendations for sleep. During the second wave, the prevalence of compliant children and adolescents for PA and ST recovered but remained below pre-pandemic levels. Furthermore, we found an association between the number of recommendations met for lifestyle behaviours during the second wave and well-being assessed between January and April 2021. In contrast, there was no such association for the periods before the pandemic or during the lockdown. Participants following the recommendations for all three lifestyle behaviours or for combinations of two of them in the second wave were more likely to report excellent health and had a higher life satisfaction score, than those not meeting any recommendation. Implications of all the available evidenceOur findings demonstrate that the COVID-19 pandemic has had a strong negative effect on childrens and adolescents lifestyle behaviours, but some recovery has taken place within the first year since the outbreak. Policymakers should imperatively consider the balance of disease prevention and promotion of a healthy lifestyle when (re-)activating restrictive measures. Given the already high prevalence of children and adolescents not meeting lifestyle recommendations in the pre-pandemic period, our findings highlight the urgent need for public health policies aiming to avoid permanent negative changes on childrens and adolescents lifestyle and to mitigate the health risks associated with adverse changes during the pandemic. In addition, our study indicates that lifestyle is an important predictor of childrens and adolescents well-being, and it further suggests that future public health strategies aiming to promote well-being should target sufficient time for PA and sleep as well as reduce ST.

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BackgroundMask wearing contributes to the reduction of transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). In Switzerland, compulsory use of masks was introduced in indoor public spaces and later in schools. In the canton of Zurich, masks were introduced for secondary school children (grades 7-9) from November 2020, and for primary school children (grades 4-6) from February 2021- along with other protective measures against SARS-CoV-2. This study explored perceptions towards the usefulness of masks in school and public in a cohort of children and adolescents in the canton of Zurich, Switzerland, in January - May 2021. MethodsSchool children aged 10 to 17 years enrolled in Ciao Corona, a prospective school-based cohort study, responded to nested online surveys between January 12 to March 24 2021 (Q1) and March 10 to May 16 2021 (Q2). Secondary school children were surveyed at Q1 and Q2, and primary school children at Q2 only. Surveys for parents and their children included questions on childrens perception of the usefulness of masks and mask wearing behavior. Associations between perceived usefulness of masks and childs school level, gender, and parents educational attainment were analyzed with Pearsons and McNemars chi-squared tests. Free-text comments provided by children were classified into categories of expressed attitude towards mask wearing. Results595 (54% girls) and 1118 (52% girls) school children responded to online questionnaires at Q1 and Q2, respectively. More than half of school children perceived masks to be useful at school (Q1:60% and Q2:57%) and in public (Q1:69% and Q2:60%). Girls perceived masks as useful more often than boys (at Q2 at school: 61% versus 53%, in public: 64% versus 57%), and children of parents with high educational attainment more often than those of parents with lower educational attainment (at Q2 at school: 61% versus 49%, in public: 63% versus 54%). There were no differences in the perceived usefulness of masks among children in primary versus secondary school. At Q1 and Q2 each, about 20% of children provided individual statements about masks, of which 36% at Q1 and 16% at Q2 reported side-effects and discomfort such as skin irritations, headache or difficulties breathing during physical education. ConclusionApproximately 60% of school children perceived masks at school and in public places as useful. A small but non-negligible proportion of children reported discomfort and side-effects that should be considered to ensure high adherence to mask wearing among school children. Trial registrationClinicalTrials.gov NCT04448717 https://clinicaltrials.gov/ct2/show/NCT04448717 CONTRIBUTION TO THE FIELD STATEMENTWorldwide about 150 countries fully closed their schools at some point during the coronavirus pandemic, while other countries - such as Switzerland - kept schools open almost all the time. However, among other protective measures, children in secondary school (aged approximately 14-16 years) had to wear masks since November 2020, and older children in primary school (aged 11-13 years) - since February 2021. As part of the large study Ciao Corona based in schools in Switzerland, we wanted to learn how children perceive the usefulness of masks in school and public. Children and their parents completed questionnaires in January-March (595 secondary school children) and March-May 2021 (1118 secondary and primary school children). We found that about 60% of children perceived masks to be useful at school and in public. Girls perceived masks as useful more often than boys, and children of parents with university or college education more often than those of parents with lower education. About 7- 9% of children reported side-effects and discomfort such as skin irritations, headache or difficulties breathing during physical education. Although side-effects were not frequently reported, they should be considered to ensure high adherence to mask wearing among school children.

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ObjectivesTo longitudinally assess severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence and clustering of seropositive children within school classes in March-April 2021 compared to June-July and October-November 2020. To examine the evolution of symptoms and the extent of under-detection of SARS-CoV-2 in children. DesignProspective cohort study of randomly selected schools and classes. SettingSchools remained open for physical attendance in Switzerland from May 2020 to the end of 2020/2021 school year. Lower school level (age range 7-10 years) and middle school level (8-13 years) children in primary schools, and upper school level (12-17 years) children in secondary schools were invited for SARS-CoV-2 serological testing in the Ciao Corona study in the canton of Zurich, Switzerland. Three testing rounds were completed in June-July 2020 (T1; after the first wave of SARS-CoV-2 infections), October-November 2020 (T2; during the peak of the second wave), and March-April 2021 (T3; after the second wave and with SARS-CoV-2 variants of concern becoming dominant). Parents completed questionnaires on sociodemographic information and symptoms. Participants2487 children (median age 12 years, age range 7-17 years) recruited from 275 classes in 55 schools participated in the testing in March-April 2021; total of 2974 children participated in at least one of the 3 testing rounds. Main outcome measuresSARS-CoV-2 serology results; clustering of seropositive children within classes; reported symptoms. ResultsThe proportion of children who were SARS-CoV-2 seropositive increased from 1.5% (95% credible interval (CrI) 0.6% to 2.6%) in June-July 2020, to 6.6% (95% CrI 4.0% to 8.9%) in October-November, and to 16.4% (95% CrI 12.1% to 19.5%) in March-April 2021. By March-April 2021, children in upper school level (12.4%; 95% CrI 7.3% to 16.7%) were less likely to be seropositive than those in middle (19.5%; 95% CrI 14.2% to 24.4%) or lower school levels (16.0%; 95% CrI 11.0% to 20.4%). Children in the upper school level had a 5.1% (95% CI -9.4% to -0.7%) lower than expected seroprevalence by March-April 2021 than those in middle school level, based on difference-in-differences analysis. The ratio of PCR-diagnosed to all seropositive children changed from 1 to 21.7 (by June-July 2020) to 1 to 3.5 (by March-April 2021). Symptoms were reported by 37% of newly seropositive and 16% seronegative children. Potential clusters of 3 or more newly seropositive children were detected in 24 of 119 (20%) classes with a high participation rate, from which a median of 17 clusters could be expected due to random distribution of seropositive children within the classes. Clustering was lowest in middle and upper school levels. Retention rate in the cohort was high (84% of T1 participants attended T3). Among participants, supporting society and research were reported more commonly for participation than personal reasons. Fear of blood sampling was the most frequently reported reason for non-participation, reported for 64% of children. ConclusionsBy March-April 2021, 16.4% of children and adolescents were seropositive in the canton of Zurich, Switzerland. The majority of clusters of SARS-CoV-2 seropositive children in school classes could be explained by community rather than intra-class transmission of infections. Seroprevalence and clustering was lowest in upper school levels during all timepoints. Trial registrationClinicalTrials.gov NCT04448717. What is already known on the topicO_LITransmission of SARS-CoV-2 in school setting largely followed community transmission in 2020. C_LIO_LIWith implemented preventive measures, secondary attack rates were low and clustering of SARS-CoV-2 infections within classes and schools (outbreaks) were observed rarely. C_LI What this study addsO_LIWith high community incidence and new variants of SARS-CoV-2, seroprevalence increased in school children between October 2020 - March 2021 in the canton of Zurich in Switzerland, and was higher in lower school levels. C_LIO_LIMost of the potential clusters of children who tested seropositive within classes could be explained by community rather than intra-class transmission of SARS-CoV-2, especially in middle and upper school levels. C_LIO_LIMore children who tested seropositive in March-April 2021 were diagnosed and reported symptoms potentially related to SARS-CoV-2 infection more frequently than those who tested seropositive in June-July or October-November 2020. C_LIO_LIThe most frequent reason for non-participation was fear of blood sampling (62% of children). C_LI

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Although long COVID in children exists, it is still unclear to what extent children are affected. The Ciao Corona study is a longitudinal cohort investigating SARS-CoV-2 seroprevalence and clustering of cases among around 2500 children and adolescents (hereafter referred to as children) from 55 randomly selected primary and secondary schools in the canton of Zurich in Switzerland. Between June 2020 and April 2021, we completed three testing phases where we collected venous blood for serological analysis and asked about symptoms with online questionnaires. We compared children who tested positive for SARS-CoV-2 antibodies in October/November 2020 with those who tested negative. Children who were seronegative in October/November 2020 and seroconverted or were not retested in March/April 2021 were excluded from the analysis (n=256). In March-May 2021 we assessed the presence of symptoms occurring since October 2020, lasting for at least 4 weeks, and persisting for either >4 weeks or >12 weeks. Overall, 1355 of 2503 children with a serology result in October/November 2020 and follow up questionnaire in March-May 2021 were included. Among seropositive and seronegative 6-to 16-year-old children, 9% versus 10% reported at least one symptom beyond 4 weeks, and 4% versus 2% at least one symptom beyond 12 weeks. None of the seropositive children reported hospitalization after October 2020. This study suggests a low prevalence of symptoms compatible with long COVID in a randomly selected population-based cohort of children followed over 6 months after serological testing.

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BackgroundSwitzerland had one of the highest incidence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections in Europe during the second wave. Schools were open as in most of Europe with specific preventive measures in place. However, the frequency and transmission of acute unrecognized, asymptomatic or oligosymptomatic infections in schools during this time of high community transmission is unknown. Thereof, our aim was to pilot a surveillance system that detects acute SARS-CoV-2 infections in schools and possible transmission within classes. Methods14 out of the randomly selected sample of the Ciao Corona cohort study participated between December 1 and 11, a time when incidence rate for SARS-CoV-2 infections was high for the canton of Zurich. We determined point-prevalence of acute SARS-CoV-2 infections of school children attending primary and secondary school. A buccal swab for polymerase chain reaction (PCR) and a rapid diagnostic test (RDT) to detect SARS-CoV-2 were taken twice 1 week apart (T1 and T2) in a cohort of children from randomly selected classes. A questionnaire assessed demographics and symptoms compatible with a SARS-CoV-2 infection during the past 5 days. ResultsOut of 1299 invited children, 641 (49%) 6- to 16-year-old children and 66 teachers from 14 schools and 67 classes participated in at least 1 of 2 testings. None of the teachers but 1 child had a positive PCR at T1, corresponding to a point-prevalence in children of 0.2% (95% CI 0.0% to 1.1%), and no positive PCR was detected at T2. The child with positive PCR at T1 was negative on the RDT at T1 and both tests were negative at T2. There were 7 (0.6%) false positive RDTs in children and 2 (1.7%) false positive RDTs in teachers at T1 or T2 among 5 schools (overall prevalence 0.7%). All 9 initially positive RDTs were negative in a new buccal sample taken 2 hours to 2 days later, also confirmed by PCR. 35% of children and 8% of teachers reported mild symptoms during the 5 days prior to testing. ConclusionIn a setting of high incidence of SARS-CoV-2 infections, unrecognized virus spread within schools was very low. Schools appear to be safe with the protective measures in place (e.g., clearly symptomatic children have to stay at home, prompt contact tracing with individual and class-level quarantine, and structured infection prevention measures in school). Specificity of the RDT was within the lower boundary of performance and needs further evaluation for its use in schools. Given the low point prevalence even in a setting of very high incidence, a targeted test, track, isolate and quarantine (TTIQ) strategy for symptomatic children and school personnel adapted to school settings is likely more suitable approach than surveillance on entire classes and schools. Trial registrationClinicalTrials.gov NCT04448717 https://clinicaltrials.gov/ct2/show/NCT04448717

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Background and aimsThe facilitating role of schools in SARS-CoV-2 infection spread is still debated and the potential of school closures to mitigate transmission unclear. In autumn 2020, Switzerland experienced one of the highest second waves of the SARS-CoV-2 pandemic in Europe while keeping schools open, thus offering a high-exposure environment to study SARS-CoV-2 infections in schools. The aim of this study was to examine longitudinal change in SARS-CoV-2 seroprevalence in children and the evolution of clustering within classes and schools from June to November, 2020, in a prospective cohort study of school children in the canton of Zurich, Switzerland. MethodsChildren from randomly selected schools and classes, stratified by district, were invited to participate in serological testing of SARS-CoV-2 in June-July and October-November 2020. Parents of children filled questionnaires on sociodemographic and health-related questions. 55 schools and 275 classes within them were enrolled, with 2603 children participating in the first, and 2552 in the second testing (age range 6-16 years). We evaluated longitudinal changes of seroprevalence in districts and investigated clustering of seropositive cases within classes and schools. ResultsOverall SARS-CoV-2 seroprevalence was 2.4% (95% CrI 1.4%-3.6%) in summer and 4.5% (95% CrI 3.2%-6.0%) in not previously seropositive children in late autumn, leading to estimated 7.8% (95% CrI 6.2%-9.5%) of ever seropositive children, without significant differences among lower, middle and upper school levels. Among the 2223 children with serology tested twice, 28 (40%) of previously seropositive were negative, and 109 (5%) previously negative became seropositive. Seroprevalence was not different between school levels or sexes, but varied across districts (1.7% to 15.0%). Between June-July and October-November 2020, the ratio of diagnosed to newly seropositive children was 1 to 8. At least one newly seropositive child was detected in 47 of 55 schools and 90 of 275 classes. Among 130 classes with high participation rate, 0, 1-2 or [≥]3 seropositive children were present in 73 (56%), 50 (38%) and 7 (5%) classes, respectively. Class level explained slightly more variation of individual serological results (standard deviation of random effects (SD) 0.97) than school level (SD 0.61) in the multilevel logistic regression models. Symptoms were reported for 22% of seronegative and 29% of newly seropositive children since summer. ConclusionsUnder a regimen of open schools with some preventive measures in place since August, clustering of seropositive cases occurred in very few classes and not across entire schools despite a clear increase in seropositive children during a period of high transmission of SARS-CoV-2. Trial registrationClinicalTrials.gov NCT04448717. https://clinicaltrials.gov/ct2/show/NCT04448717

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Importance: Understanding transmission and impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in school children is critical to implement appropriate mitigation measures. Objective: To determine the variation in SARS-CoV-2 seroprevalence in school children across districts, schools, grades, and classes, and the relationship of SARS-CoV-2 seroprevalence with self-reported symptoms. Design: Cross-sectional analysis of baseline measurements of a longitudinal cohort study (Ciao Corona) from June-July 2020. Setting: 55 randomly selected schools and classes stratified by district in the canton of Zurich, Switzerland (1.5 million inhabitants). Participants: Children, aged 6-16 years old, attending grades 1-2, 4-5 and 7-8. Exposure: Exposure to circulating SARS-CoV-2 between February and June 2020 including public lock-down and school closure (March 16-May 10, 2020). Main Outcomes and Measures: Variation in seroprevalence of SARS-CoV-2 in children across 12 cantonal districts, schools, and grades using a Luminex-based antibody test with four targets for each of IgG, IgA and IgM. Clustering of cases within classes. Analysis of associations of seropositivity and symptoms. Comparison of seroprevalence with a randomly selected adult population, based on Luminex-based IgG and IgA antibody test of Corona Immunitas. Results: In total, 55 schools and 2585 children were recruited (1337 girls, median age 11, age range 6-16 years). Overall seroprevalence was 2.8 % (95% CI 1.6-4.1%), ranging from 1.0% to 4.5% across districts. Seroprevalence was 3.8% (1.9-6.1%) in grades 1-2, 2.5% (1.1-4.2%) in grades 4-5, and 1.5% (0.5-3.0%) in grades 7-8. At least one case was present in 36/55 tested schools and in 43/128 classes with [≥]50% participation rate and [≥]5 children tested. 73% of children reported COVID-19 compatible symptoms since January 2020, but none were reported more frequently in seropositive compared to seronegative children. Seroprevalence of children was very similar to seroprevalence of randomly selected adults in the same region in June-July 2020, measured with the same Corona Immunitas test, combining IgG and IgA (3.1%, 95% CI 1.4-5.4%, versus 3.3%, 95% CI 1.4-5.5%). Conclusions and Relevance: Seroprevalence was inversely related to age and revealed a dark figure of around 90 when compared to 0.03% confirmed PCR+ cases in children in the same area by end of June. We did not find clustering of SARS-CoV-2 seropositive cases in schools so far, but the follow-up of this school-based study will shed more light on transmission within and outside schools. Trial registration: ClinicalTrials.gov Identifier: NCT04448717, registered June 26, 2020. https://clinicaltrials.gov/ct2/show/NCT04448717

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IntroductionSeroprevalence and transmission routes of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection in children and adolescents, especially in school setting, are not clear. Resulting uncertainty is reflected in very different decisions on school closures and reopenings across countries. The aim of this longitudinal cohort study is to assess the extent and patterns of seroprevalence of SARS-CoV-2 antibodies in school-attending children repeatedly. It will examine risk factors for infection, relationship between seropositivity and symptoms, and temporal persistence of antibodies. Additionally, it will include testing of school personnel and parents. Methods and analysisThe study (Ciao Corona) will enroll a regionally representative, random sample of schools in the canton of Zurich, where 18% of the Swiss population live. Children aged 5 to 16 years, attending classes in primary and secondary schools are invited. Venous blood and saliva samples are collected for SARS-CoV-2 serological testing after the first wave of infections (June/July 2020), in fall (October/November 2020), and after winter (March/April 2021). Venous blood is also collected for serological testing of parents and school personnel. Bi-monthly questionnaires to children, parents and school personnel cover SARS-CoV-2 symptoms and tests, health, preventive behavior, lifestyle and quality of life information. Total seroprevalence and cumulative incidence will be calculated. Hierarchical Bayesian logistic regression models will account for sensitivity and specificity of the serological test in the analyses and for the complex sampling structure, i.e., clustering within classes and schools. Ethics and disseminationThe study was approved by the Ethics Committee of the Canton of Zurich, Switzerland (2020-01336). The results of this study will be published in peer-reviewed journals and will be made available to study participants and participating schools, the Federal Office of Public Health, and the Educational Department of the canton of Zurich. Trial registration number NCT04448717. O_LICiao Corona is a large, prospective school-based cohort study and will provide robust data on severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) seroprevalence, transmission routes and immunity over time in a representative sample of school children. C_LIO_LIThe longitudinal design will allow describing temporal trends of immunity to SARS-CoV-2 infection and to evaluate effects of school structure and preventive measures. C_LIO_LIThis study will inform goal-oriented policy decisions in school management during subsequent outbreaks. C_LIO_LIParticipation bias, missing questionnaires, desirability bias, and loss of follow up may occur. The validity of serological tests may also hamper results. C_LI

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PURPOSE: In obesity prevention, understanding psychosocial influences in early life is pivotal. Reviews reported contradictory results and a lack of longitudinal studies focusing on underlying lifestyle factors. This study tested whether psychosocial Quality-Of-Life (QOL) was associated with pre-schoolers' lifestyle and adiposity changes over one school year and whether lifestyle moderated the latter. It was hypothesised that QOL might not impact adiposity in everybody but that this might depend on preceding lifestyle. METHOD: Longitudinal data from 291 Swiss pre-schoolers (initially 3.9-6.3 years) was available. The following measures were used in longitudinal regressions: psychosocial QOL by PedsQL, adiposity (BMI z-score, waist, fat%), diet (food frequency), sedentary time and accelerometer-based activity. RESULTS: Concerning lifestyle, low psychosocial QOL was only related to unfavourable changes in diet (less fruit ß = 0.21 and more fat intake ß = -0.28) and lower physical activity (ß = 0.21). Longitudinal QOL-adiposity relations appeared only after moderation by lifestyle factors (beta-range 0.13-0.67). Low psychosocial QOL was associated with increased adiposity in children with an unhealthy diet intake or high sedentary time. By contrast, low psychosocial QOL was associated with decreasing adiposity in high fruit consumers or more physically active pre-schoolers. CONCLUSION: Results emphasise the need for testing moderation in the QOL-adiposity relation. An unhealthy diet can be a vulnerability factor and high physical activity a protective factor in QOL-related adiposity. Consequently, QOL and lifestyle should be targeted concurrently in multi-factorial obesity prevention. The environment should be an 'activity encouraging, healthy food zone' that minimises opportunities for stress-induced eating. In addition, appropriate stress coping skills should be acquired.

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