Based on studies in China, Iceland, the Netherlands and Italy it seems that children are less affected by SARS-CoV-2 infections and play a lesser role in the dissemination of the SARS-CoV-2 virus. It is unclear to which extend this is due to lesser exposure or an inherent decreased susceptibility to become infected. The low reported number of cases in children can be partially explained by the lower testing rates in the pediatric population.

To get insight in the transmission of SARS-CoV-2 virus in school-aged children it is necessary to compile data on infection of SARS-CoV-2 virus in the child and (pre-)adolescent population. The main objective of this study is to determine the sero-prevalence and sero-conversion of antibodies against SARS-CoV-2 in a sample of school-aged children (primary and secondary school) in Belgium at different time points. Additionally this study will gain insight in the incidence of SARS-CoV-2, the proportion of asymptomatic infections with SARS-CoV-2 and the role of COVID-19 infection in household members. It will give us the possibility to investigate potential risk factors for infection.

The study population includes primary and secondary school children from two predefined age groups (8-9 and 13-14 years old). These two groups are chosen because studies have shown more pronounced symptoms of infection and disease depending on age and because transmission dynamics also vary by age. The study population will be recruited in all Belgian schools including Brussels-Capital, Flemish and Walloon Region (including the German speaking community).

The study is a prospective cohort study. In total there will be 3 testing points with 3 months between each point. The sample size calculation indicated the inclusion of 41 randomly selected primary and 41 secondary schools and in each school 20 randomly selected pupils of the defined age groups. Parents/legal child caregiver will complete a questionnaire at each of the testing points providing basic socio-demographic characteristics (at baseline only) and risk-behavior and health characteristics including presence of symptoms during the time since the previous testing point (at each of the testing points). The questionnaire will be completed through a secured online application during the same week as the child had the saliva sample for serological testing taken.

Saliva samples will be sent to the Sciensano laboratory for serological analysis. The child's parent/legal caregiver will be informed of the serological result.

Study data will guide the Belgian policy makers in their advices to limit circulation of SARS-CoV-2 in Belgium and more specifically in their advice on measures for school-age children. This data might also be used to guide health authorities that have to decide on vaccination strategies once a vaccine is available; to decide whether children should be included for vaccination to create herd immunity or not.

Inclusion Criteria:

• Primary schools: any pupil from the 3rd grade (or from the 2nd grade if not enough participants from 3rd grade are available in the selected school) who regularly attend school and is present at school at the first testing point
• Secondary schools: any pupil from 2nd grade who regularly attend school and is present at school at the first testing point
• Prior known SARS-CoV-2 infection does not exclude participation

Exclusion Criteria:

• Pupils who refuse to have the saliva sample taken
• Being a sibling of a study participant
• No informed consent form signed by parent of legal caregiver available at the recruitment

The study population includes primary and secondary school children from two predefined age groups. These two groups are chosen because studies have shown more pronounced symptoms of infection and disease depending on age and because transmission dynamics also vary by age. Adolescents of 15 year and older will not be included as their symptom patterns resembles more disease pathology of adults. Because of pragmatic and practical reasons we will include primary school children from the 3rd grade (age 8-9) and secondary schoolchildren from the 2nd grade (age 13-14).

The study sample will be recruited in all Belgian schools including Brussels-Capital, and the Flemish and Walloon Regions (including the German speaking community). School attendance in this age group is high in Belgium, because of this, recruitment of school-age children through schools is acceptable.