Discussion
MAY, SECONDARY, HOLIDAY
Apart from some ventilation and physical distancing measures, most IPC measures were implemented by more than 60% of primary and secondary schools. Overall, the implementation of IPC measures was similar in primary and secondary schools. However, since reopening in May 2020, more primary than secondary schools reported temporary closures due to the detection of COVID-19. Schools with poorer IPC measure implementation usually observed a higher prevalence of SARS-CoV-2 antibodies among pupils and staff. However, this association was only statistically significant for the assessment of all IPC measures together in the population compromised of pupils and staff. Despite observing comparable effect sizes in pupils and staff separately, statistically significant observations were not observed in analyses of neither these subpopulations nor the trends for the three different IPC measure subcategories.The higher number of school closures observed in primary, compared to secondary schools, might be due to the general recommendation and infrastructure available for organizing distance learning in secondary, but not in primary schools. Secondary schools could more easily adjust to providing distance learning, while this option was more difficult to implement at primary schools. The general recommendation for secondary schools in Belgium was to organize distance learning for half of the school population for grades 3 and higher [Most schools reported thorough application of hygiene measures, possibly as a result of the widespread attention for hand hygiene, disinfection and cleaning from the onset of the pandemic [Mask wearing was reported to be almost fully implemented by primary and secondary school staff and by secondary school pupils. Although we could not assess the effect of mask wearing on SARS-CoV-2 prevalence in our study, findings in kindergarten and primary schools in Georgia (USA) suggest that this influences the transmission of SARS-CoV-2 in school settings [Our study identified a statistically significant association between the prevalence of anti-SARS-CoV-2 antibodies and the implementation of IPC measures for the population compromised of pupils and staff together but not for pupils and staff separately. Since effect sizes, expressed as aRR, are of the same magnitude in both groups, this is probably the result of a loss of statistical power. Compliance with recommendations regarding IPC measures might thus be beneficial for the total school population. Results for the different subcategories of IPC measures point in the same direction, but effect sizes are notably smaller and not statistically significant. Studies do suggest that ventilation measures might be more effective [A strength of our study is the random selection of a geographically and socially proportional sample of pupils and staff from Belgian schools. Also, this study is the first to assess the implementation of IPC measures in schools in relation to the prevalence of anti-SARS-CoV-2 antibodies among pupils and staff in Belgium. There are also limitations. The data on the implementation of IPC measures were self-reported which is prone to information bias and misinterpretation. Additionally, data collection started in December 2020 and covered the preceding months which could result in recall bias. Moreover, the first testing period covered a rather heterogeneous phase of the pandemic including periods of in-person teaching and the summer holiday in July and August 2020. Another limitation is that the individual characteristics of the participants were not included in our analyses, and that the reverse causality of outbreaks and implementation of IPC could not be addressed. Furthermore, we adjusted for community exposure in our analysis of the impact of IPC measures on the prevalence of anti-SARS-CoV-2 antibodies in schools. This might have impacted the estimated effect of the IPC measures if the community prevalence was affected by the IPC measures.. In addition, we did not adjust the seroprevalence for test specification (sensitivity and specificity). Finally, while representative for Belgian schools, the sample sizes at the school level are small and do not allow to quantify the attributable impact of individual IPC measures.