Implementation of IPC measures 
The implementation of IPC measures in Belgian schools is presented using data from three data collection periods (December 2020/January 2021, March 2021 and May/June 2021).IPC measures were grouped according to the target age as: [For each data collection period, we reported the number and proportion of schools that implemented each IPC measure. For school and class closures, we reported the number and proportion of schools with at least one closure during the assessed period. The other IPC measures were assessed using a 5-point Likert scale ranging from 1 (the IPC measure was ‘not applied at all’) to 5 (the IPC measure was ‘fully applied‘). For the analysis, we considered measures with a score of 4 or 5 as ‘implemented’ and with a score from 1 to 3 as ‘not implemented’. When a school did not respond for a specific measure (Table S2, S3, Supplement 