The investigators hypothesize that children are less likely to become infected in the school environment, compared to the household or outside the school setting. Primary schools do not play a major role in SARS-CoV-2 spreading. They hypothesize that incidence level in schools is mainly a consequence of community transmission. The incidence level in household and that the secondary attack rate in schools remains at least 5 times lower that the transmission level in households.

Inclusion Criteria:

• All children and school attenders from selected schools was invited to participate

Exclusion Criteria:

• Refusal to provide written informed consent before enrolment

All Belgian primary schools of the Federation Wallonia - Brussels that have organized a full resumption of school activities with the control measures recommended by the government, are eligible for the study.

Twelve schools were chosen by purposive criteria: either high (≥2 classes per grade) or low size of the school, a school within an area with either a high or a low incidence of SARS-CoV-2 during April 2020, the first wave of covid-19 in Belgium, and a school with either a high or a low socioeconomic level (Belgian 20-point-scale ISE idex >14 or <6).

All collected IPD