Myopericarditis is a rare complication to messenger ribonucleic acid (mRNA) COVID-19 vaccines, especially in male adolescents and young adults. The risk in children 5-11 years old is unknown. In Denmark, the Pfizer-BioNTech mRNA COVID-19 vaccination was recommended from December 1, 2021 in individuals aged 5-11 years old. We aim to estimate the incidence of myopericarditis in children 5-11 years old after mRNA COVID-19 vaccination among vaccinated individuals based on a nationwide prospective population-based cohort study with detailed clinical phenotyping.

AIM The project seeks to estimate the incidence of myopericarditis in children 5-11 years old after mRNA COVID-19 vaccination.

HYPOTHESIS The incidence of myopericarditis after mRNA COVID-19 vaccination in children 5-11 years old is significantly lower than the incidence in adolescents.

METHOD Prospective nationwide population-based cohort study of all individuals aged 5-11 years hospitalized due to myocarditis and pericarditis after COVID-19 mRNA vaccination in the period December 1, 2021 to February 1, 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 vaccine-associated disease.

To calculate the incidence of myopericarditis among Danish individuals 5-11 years old, the number of individuals who have been mRNA COVID-19 vaccinated will be attained from the National COVID-19-vaccine Database at the Statens Serum Institut.

Inclusion Criteria: Children 5-11 years old hospitalized due to myocarditis and pericarditis following mRNA vaccination

Exclusion Criteria: Viral or bacterial cause of myopericarditis, or other known cause of myopericarditis

All children 5-11 years old in Denmark who have received mRNA vaccination