#2-77E. Vaccine Safety Confirmed: Assessing Myositis Risk After SARS-CoV-2 Immunization (Rheum, 2025)

How should clinicians address patient concerns regarding the safety of SARS-CoV-2 vaccines and the potential triggering of rare autoimmune disorders like idiopathic inflammatory myopathies, or myositis? This critical question is addressed by a large population-based cohort study conducted in Norway and Sweden, analyzing over 13 million individuals to characterize the risk of incident myositis following mRNA or adenoviral vector vaccination. The study followed participants for over 13.6 million person-years, comparing myositis rates in 180-day risk periods post-vaccination with unvaccinated periods. The major finding is that neither mRNA vaccines nor the adenoviral vector vaccine were associated with an overall increased risk of myositis compared to unvaccinated periods, providing robust reassurance (adjusted IRRs for 180-day risk periods were 0.84 and 1.31, respectively). However, due to the rarity of myositis events (only 99 mRNA-related events and 13 adenoviral vector-related events), the study’s statistical power was inherently limited for detailed analyses by specific vaccine product or myositis subtype. This large-scale, register-based study offers physicians vital evidence derived from comprehensive national data, enabling confident and data-driven risk communication about the safety profile of COVID-19 vaccines regarding inflammatory myopathies in daily practice46.Citation: Ljung R et al. SARS-CoV-2 vaccination and myositis in Norway and Sweden. Rheumatology. 2025.

DOI: 10.1093/rheumatology/keaf609

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