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

The objective of this study was to characterize the risk of myositis after Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccination with messenger ribonucleic acid (mRNA) or adenoviral vector vaccines.

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 the start of follow-up on 27 December 2020. The participants were followed until incident diagnosis of myositis, censoring, or the end of the study (21 May 2023). We used Poisson regression to estimate incidence rate ratios (IRRs) with 95% CIs, comparing rates in risk periods (day 0 up to and including day 180 after each dose) with rates in unvaccinated periods. Corrections for potential confounding were made by adjusting for sex, age, occupation (health-care worker), region, positive COVID-19 test, and comorbidities.

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). The adjusted IRRs for the 180-day risk periods following any combination of mRNA vaccines and the adenoviral vector vaccine were 0.84 (95% CI, 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.

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.