Multisystem inflammatory syndrome in children (MIS-C) is a rare severe complication to SARS-CoV-2 infection in children. It has been reported to occur in approximately one of 3,000 to 4,000 unvaccinated children during the COVID-19 waves dominated by variant Alpha of SARS-CoV-2. However, the incidences following SARS-CoV-2 infection with variant delta and omicron, as well as among vaccinated children and adolescents with SARS-CoV-2 is yet unexplored.

The investigators aim to estimate the incidence of MIS-C and 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, based on a nationwide prospective population-based cohort study.

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.

Inclusion Criteria:

• MIS-C according to criteria devised by the World Health Organization and Centers for Disease Control and Prevention (CDC)
• Invasive bacterial infections and viral infections secondary to SARS-CoV-2 or as consequence of SARS-CoV-2 lock down

Exclusion Criteria:

-

All children and adolescents in the whole population of Denmark