Data collection 
The local study coordinator – a staff member of the participating school – collected the data on the implementation of IPC measures via a secured online questionnaire using the ‘LimeSurvey’ platform (LimeSurvey version 3.22.24 + 2,000,630; see Supplement During each data collection period, the prevalence of anti-SARS-CoV-2 antibodies among the same pupils and staff of the participating schools was assessed using saliva samples. This technique was validated in a pilot study in the province of Limburg [