Children are relatively spared from the direct clinical impact of COVID-19. Pediatric patients with regular follow-up at tertiary pediatric centres and presenting one or more chronic conditions might be at increased risk for severe COVID-19. With a prospective seroprevalence study, we aim to study COVID-19 incidence, disease course and risk factors associated with SARS-CoV-2 infection in this specific context.

Children and adolescents seem to be relatively spared from severe SARS-CoV-2 infection. Throughout the course of the COVID-19 pandemic, these findings were confirmed with, for example, also in Belgium only 1.8% of the confirmed infections, 1.6% of the hospitalizations in intensive care, and 0.011% of the deaths from COVID-19 attributed to children by the end of May 2020. Multiple immunological hypotheses (ACE2 expression, lymphocyte repertoire, cross immunization) have been suggested without one clear explanation to date. In addition, children seem to play a limited role in the spread of the virus and are not considered the "engine of the epidemic".

Population seroconversion studies have been initiated with the determination of antibodies to SARS-CoV-2. For Belgium, seroconversion rates were estimated at around 2.1% at the end of March, 4.3% in mid-April, 6% at the beginning of May and 6.9% at the end of May 2020, comparable to international studies. However, even with these prospective studies, it remained unclear whether the seroconversion rate in children is comparable to adults. In addition, it has not yet been demonstrated whether the mild course and limited hospitalizations in children are related to either a mild course or (in part) also to a lack of exposure to SARS-CoV-2 and therefore limited seroconversion in children.

With this study we aim to monitor both the presence of antibodies (serology) in children with chronic disease, followed in a tertiary hospital in Belgium, combined with the registration of the clinical course of COVID-19 in this population. In addition, we want to correlate the social activities with the risk of exposure to the virus and subsequent seroconversion. Blood samples for serology are performed at a 3- to 6-month interval, linked to an already planned blood sample for diagnostic reasons. This data will allow to gain more insight into the actual rate of infection and the symptoms caused by COVID-19 in children with a chronic disease, on the one hand, and, on the other hand, how the social behavior, limited by the lockdown measures or after its progressive relaxation, affects seroconversion in these pediatric populations. These data may or may not confirm current hypotheses about infection rate and clinic severity and will also influence current and future policies such as closing schools and defining at-risk populations.

Inclusion Criteria:

• children <18 years of age
• a clearly defined chronic condition requiring follow-up at the tertiary pediatric hospital of the Ghent University Hospital
• routine blood sampling planned per 3 to 6 months

Exclusion Criteria:

• no clear diagnosis or reason for follow-up at the Ghent University Hospital
• no routine blood sampling planned
• unable to understand or provide informed consent

• children <18 years of age
• a clearly defined chronic condition requiring follow-up at the tertiary pediatric hospital of the Ghent University Hospital