SARS-CoV-2 vaccination and myositis in Norway and Sweden

Objectives To characterize the risk of myositis after SARS-CoV-2 vaccination with mRNA or adenoviral vector vaccines. Methods We conducted a population-based cohort study in Norway and Sweden of 13 million persons who turned 12 years or older in 2021 and were residents at start of follow-up on December 27, 2020. Persons were followed until incident diagnosis of myositis, censoring, or end of study (May 21, 2023). We used Poisson regression to estimate incidence rate ratios (IRRs) with 95% confidence intervals (CIs), comparing rates in risk periods (day 0 up to and including day 180 after each dose) with unvaccinated periods. Corrections for potential confounding were made by adjusting for sex, age, occupation (healthcare worker), region, positive covid-19 test, and comorbidities. Results We observed 101 myositis events in 7,002 398 unvaccinated person-years and 99 myositis events within 180 days of any combination of mRNA vaccines (6,241 529 person-years), and 13 with the adenoviral vector vaccine (445 256 person-years). Adjusted IRRs for the 180-day risk periods following any combination of mRNA vaccines and the adenoviral vector vaccine were 0.84 (95% confidence interval, 0.63–1.11) and 1.31 (0.72–2.36) respectively, compared with unvaccinated periods. The estimates for each specific first, second and third dose of mRNA were similar to the estimate of all doses combined. Conclusion In this nationwide study of > 13.6 million person-years in two countries there were no signs of an increased risk of myositis after SARS-CoV-2 vaccination, neither after the mRNA vaccines nor after the adenoviral vector vaccine.