A large national registry study from Norway and Sweden has revealed no increased risk of myositis after COVID-19 vaccination. Among about 7 million person-years of vaccine exposure, nearly 100 myositis cases were observed, a rate similar to that of unvaccinated individuals. This held for both mRNA and adenoviral vector vaccines, contradicting earlier case reports that suggested a possible link. The study was published in Rheumatology by Rickard L. and colleagues.
The primary aim was to characterize the risk of myositis after vaccination with mRNA vaccines against SARS-CoV-2 or adenoviral vector vaccines by comparing post-vaccination risk periods to unvaccinated time periods. This was a population-based cohort study conducted across Norway and Sweden, including approximately 13 million individuals aged 12 years or older who were residents at the start of follow-up on December 27, 2020. Participants were followed until the first diagnosis of myositis, censoring, or the end of the study period on May 21, 2023. The analysis included the use of Poisson regression to calculate adjusted incidence rate ratios (IRRs) with 95% confidence intervals, comparing myositis rates during predefined risk periods (day 0 to 180 after each vaccine dose) with unvaccinated periods. Adjustments were made for sex, age, healthcare worker status, region, prior positive COVID-19 test, and comorbidities in an effort to minimize confounding.
Key findings
A total of 101 myositis cases were recorded during 7,002,398 unvaccinated person-years, 99 cases occurred within 180 days of any combination of mRNA vaccine exposure over 6,241,529 person-years, and 13 cases were noted after adenoviral vector vaccination over 445,256 person-years.
The adjusted IRR for myositis following any mRNA vaccine dose was 0.84 (95% CI, 0.63–1.11), while the adjusted IRR following adenoviral vector vaccination was 1.31 (95% CI, 0.72–2.36), both compared with unvaccinated periods.
Neither of these estimates suggested a significantly increased risk.
Notably, similar IRRs were found when the analysis was stratified by first, second, and third doses of mRNA vaccines, supporting consistency across dosing schedules.
This large population-based study demonstrated that vaccination against SARS-CoV-2, both with mRNA and adenoviral vector vaccines, was not associated with an increased risk of myositis.
Reference:
Rickard Ljung, Nicklas Pihlström, Jesper Dahl, German Tapia, Anders Sundström, Marja-Leena Nurminen, Ingrid E Lundberg, Øystein Karlstad, Marie Holmqvist, Nils Feltelius, SARS-CoV-2 vaccination and myositis in Norway and Sweden, Rheumatology, 2025;, keaf609,
https://doi.org/10.1093/rheumatology/keaf609
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