AIM The project seeks to estimate the incidences of MIS-C in vaccinated and unvaccinated
children and adolescents following variants delta and omicron, and other complications of
SARS-CoV-2 including complication due to SARS-CoV-2 lock-down, .

HYPOTHESIS The incidences of MIS-C following SARS-CoV-2 variant delta and omicron are
similar to the risk following variant alpha in unvaccinated children and adolescents,
while the incidences among vaccinated children and adolescents are significantly lower.
Further, severe complications to SARS-CoV-2, e.g. PVL producing S aur. infection is
increased, as well as severe RSV-complication in older children in the post-COVID-lock
down RSV epidemic

METHOD Prospective nationwide population-based cohort study of all individuals aged 0-17
years hospitalized due to MIS-C, or other complications to SARS-CoV-2 in the period
August 15, 2021 to August 14, 2022. The setting is a multicenter study including all 18
Danish Pediatric Departments, providing 24 hours emergency service, and in- and
out-patient treatment for all Danish inhabitants aged ≤ 17 years. As part of a pediatric
nationwide COVID-19 research set-up, all 18 departments have a principal investigator
responsible for prospective real-time data collection of patients with MIS-C and other
complications to SARS-CoV-2, including complications secondary to the lock-down.

To calculate the incidence of MIS-C among Danish children and adolescents, the number of
individuals who has tested positive for a SARS-CoV-2 infection in the period July 15 2021
to July 14 2022 will be attained from the National COVID-19-vaccine Database at the
Statens Serum Institut. From the Statens Serum Institut and the Danish Microbiological
Departments, the numbers bacterial infections including PVL-producing S aureus
bacteremias and RSV infection will be obtained.