A study published by The BMJ provides more reassuring data on the risk of heart inflammation (myocarditis or myopericarditis) after a COVID-19 vaccination.
It confirms previous reports of increased risk after COVID-19 vaccination with BNT162b2 (Pfizer-BioNTech) and mRNA-1273 (Moderna) but shows that the absolute number of cases were low, even in younger age groups, providing further evidence to support the overall safety of mRNA vaccines for COVID-19.
Myocarditis (inflammation of the heart muscle) and myopericarditis (inflammation of the outer lining of the heart) are rare but serious conditions, usually triggered by a viral, bacterial or fungal infection.
Recent reports and studies have indicated an increased risk of heart inflammation after mRNA COVID-19 vaccination, particularly after the second dose. But as of yet, no study has investigated the association using information from a complete population.
The link between COVID-19 vaccination and heart conditions
Researchers in Denmark used national healthcare data to look for links between the mRNA covid-19 vaccination and a hospital diagnosis of myocarditis or pericarditis, increased blood troponin levels (a measure of myocardial damage) and a hospital stay lasting more than 24 hours.
They found that nearly five million Danish residents aged 12 years or older received either the Pfizer-BioNTech or Moderna COVID-19 vaccination.
Participants were monitored from 1 October 2020 to 5 October 2021, and a range of potentially influential factors were taken into account, such as age, sex, vaccine priority group, and underlying health conditions.
During follow-up, 269 participants developed myocarditis or myopericarditis, of whom 108 (40%) were 12-39 years old and 196 (73%) were male.
Overall, the results showed a strong association between vaccination with Moderna and myocarditis or myopericarditis, while COVID-19 vaccination with Pfizer-BioNTech was only associated with an increased rate of myocarditis or myopericarditis among women.
The absolute rate is low
The rate of myocarditis or myopericarditis was higher for Moderna vaccination than for Pfizer–BioNTech vaccination. But the absolute number of events after either COVID-19 vaccine was low, and cases were predominantly mild.
For example, of 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.
Amongst women, the absolute rate was two per 100,000, and in men, it was 6.3 per 100,000. Among 12–39-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 around a 50% reduced risk of cardiac arrest or death (the most severe manifestations of myocarditis or myopericarditis) compared with unvaccinated individuals.
In 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.
This is an observational study; therefore, it can’t establish cause. The researchers pointed to some potential sources of bias, such as increased public awareness of the potential side effects of COVID-19 vaccines, which may have affected the results.
However, they say this was a well-designed study based on high-quality healthcare data for a complete population, and results were largely unchanged after additional analyses, suggesting that they withstand scrutiny.
The researchers concluded that mRNA COVID-19 vaccination with Moderna and Pfizer-BioNTech is associated with an increased risk of myocarditis or myopericarditis in the Danish population, but the absolute rate after either vaccine was low, even in younger age groups.