Previous reports based on individual cases have suggested that myositis can be a rare side effect of Covid-19 vaccines, but a study of national health registry data in Norway and Sweden found roughly only 100 myositis cases occurring over some 7m person-years of Covid vaccine exposure.
This rate was not significantly different from that seen among unvaccinated people during the same timeframe, the scientists said, and this was true for both the mRNA and adenoviral vector vaccines used in these countries.
Myositis is a rare disease characterised by inflammation of the muscles, which can cause weakness, pain, and fatigue
The public health researchers said the results of their controlled study contradicted previous reports of the condition occurring in the wake of immunisation with the mRNA and adenoviral vector vaccines, reports Medpage Today.
And myositis cases that did arise were extremely rare, with just 112 events recorded during 7m person-years of observation, according to Rickard Ljung, MD, PhD, MPH, of the Swedish Medical Products Agency in Uppsala, Sweden, and colleagues.
Against those 112 episodes, national health registry records showed 101 occurring in a similar number of unvaccinated persons over the same time frame, the group reported in Rheumatology.
Ljung and colleagues calculated rate ratios of 0.84 (95% CI 0.63-1.11) for the Pfizer-BioNTech and Moderna mRNA vaccines, and 1.31 (95% CI 0.72-2.39) for the less widely-used AstraZeneca adenoviral vector product, after adjusting for age, sex, healthcare occupation, region, comorbidities, and positive Covid-19 test.
“In this nationwide study of more than 13m people 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,” they wrote.
Several dozen cases of inflammatory myositis seen after Covid-19 vaccination have been reported in medical journals, and FDA-approved labels for the two mRNA products list “new or worsened muscle pain” among the potential adverse effects.
However, no previous studies have compared rates of myositis in vaccinated versus unvaccinated people – which essentially demand population-based analysis, given the acknowledged rarity of such events.
For the current study, Ljung and colleagues searched registry data in Norway and Sweden, from December 2020 to May 2023, identifying cases of myositis diagnosed within 180 days of receiving Covid-19 vaccines, or a corresponding index date for unvaccinated persons.
Just more 7m person-years of data were analysed for the unvaccinated control group, more than 6.2m person-years for people receiving mRNA vaccines, and about 450 000 person-years for those vaccinated with the adenoviral vector product. Data for the two countries were pooled to allow meaningful rate-ratio calculations.
Because the three vaccines were all supposed to be administered in multiple doses, Ljung and colleagues examined myositis cases occurring after one, two, and three mRNA vaccine doses, and after one and two doses of the AstraZeneca vaccine. None of these analyses showed a significant increase in risk with vaccination, and there was no substantial numerical increase either.
Also absent was any suggestion of increased risk within 90 days of vaccination, or during the final five months of the six-month follow-up period.
Limitations to the study included its reliance on administrative data, such that there was “no possibility to undertake any case ascertainment nor assess time from start of symptoms to definite diagnosis”, the researchers noted.
It was also possible that episodes identified in the records actually reflected pre-existing myositis. Other potential confounders, not addressed in the study, included the potential for Covid-19 itself to cause myositis, and for certain “antibody groups or phenotypic groups” to be more or less susceptible than others to myositis.
Study details
SARS-CoV-2 vaccination and myositis in Norway and Sweden
Rickard Ljung, Nicklas Pihlström, Jesper Dahl et al.
Published in Rheumatology on 22 November 2025
Abstract
Objectives
To characterise 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 13m persons who turned 12 or older in 2021 and were residents at start of follow-up on December 27, 2020. They 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 > 13.6m 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.
Rheumatology article – SARS-CoV-2 vaccination and myositis in Norway and Sweden (Open access)
Medpage Today article – Scandinavian Study Shreds COVID Vaccine-Myositis Link (Open access)
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