Participating schools
SECONDARY, RECRUITMENT
We contacted 98 primary and 108 secondary schools of which 44 primary and 40 secondary schools agreed to participate. Of these 84 schools, 45 belonged to the Dutch and 39 to the French language network. All but one school (Eighty-one schools (43 primary and 38 secondary) fulfilled the requisite 10 (out of a total of 14) assessments of IPC measure implementation and were included in the analysis. Among these, 21 (26%) schools were classified as implementing the measures thoroughly, 37 (46%) as moderately and 23 (28%) as poorly. The mean prevalence of anti-SARS-CoV-2 antibodies for each group was calculated based on a total of 1,285 pupils (710 primary, 575 secondary school pupils) and 818 staff (432 primary, 386 secondary school staff). More details on sample recruitment and participation can be found in Figure S4 (Supplement 