The risk of myocarditis after an mRNA COVID-19 vaccination is rare, according to a study reported today.
Researchers in Denmark used national healthcare data to look for links between mRNA vaccination and a hospital diagnosis of myocarditis or pericarditis, increased blood troponin levels and a hospital stay of 24 hours or more.
The analysis, reported in The BMJ, included data from nearly five million Danish residents aged 12 years and older who received either the Pfizer-BioNTech or Moderna vaccine.
Participants were monitored from 1, October 2020, to 5, October 2021, and during the follow-up, 269 participants developed myocarditis or myopericarditis, of whom 108 (40%) were 12-39 years old and 196 (73%) were male.
While the results show a strong association between the Moderna vaccine and myocarditis or myopericarditis, and vaccination with Pfizer-BioNTech was associated with an increased rate of myocarditis or myopericarditis among women, the absolute number of events after either vaccine was low, and cases were predominantly mild.
The research found of the 3,482,295 individuals vaccinated with Pfizer-BioNTech, 48 developed myocarditis or myopericarditis within 28 days of vaccination (an absolute rate of 1.4 per 100,000) compared with unvaccinated individuals.
Among women, the absolute rate was 1.3 per 100,000 and in men it was 1.5 per 100,000. Among 12 to 39 year olds, the absolute rate was 1.6 per 100,000, and in the 12 to 17 year age group it was one per 100,000 within 28 days of receiving the Pfizer-BioNTech vaccine.
Out of the 498,814 individuals who had the Moderna vaccine, 21 developed myocarditis or myopericarditis within 28 days of vaccination (an absolute rate of 4.2 per 100,000) compared with unvaccinated individuals.
In women, the absolute rate was two per 100,000 and in men it was 6.3 per 100,000. Among 12 to 39 year olds, the absolute rate was 5.7 per 100,000 within 28 days of receiving the Moderna vaccine.
Both vaccines were also associated with about a 50% reduced risk of cardiac arrest or death compared with unvaccinated individuals.
By contrast, there was a 14-fold increased risk of cardiac arrest or death 28 days after a positive COVID-19 test compared with uninfected individuals.
Although the researchers say they cannot establish cause and they also point to some potential sources of bias, say the results were largely unchanged after additional analyses, suggesting that they withstand scrutiny.
Husby A, Hansen JV, Fosbøl E et al. SARS-CoV-2 vaccination and myocarditis or myopericarditis: population based cohort study. BMJ 17 December 2021; doi: 10.1136/bmj-2021-068665
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