Low chance of heart problems after Covid-19 mRNA vaccine

The number of people who developed myocarditis or myopericarditis after receiving a Covid-19 mRNA vaccine is low – 1.4 persons per 100,000 for Pfizer/BioNTech and 4.2 persons per 100,000 for Moderna. — AFP Filepic

A study, published by The BMJ on Dec 16 (2021), provides more reassuring data on the risk of heart inflammation (myocarditis or myopericarditis) after the administration of mRNA (messenger ribonucleic acid) vaccines to protect against the SARS-CoV-2 virus.

It confirms previous reports of an increased risk after vaccination with the Pfizer/BioNTech and Moderna vaccines, but shows that the absolute number of cases was 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 that are usually triggered by a viral, bacterial or fungal infection.

Recent reports and studies have indicated an increased risk of heart inflammation after mRNA vaccination, particularly after the second dose.

But as yet, no study has investigated the association using information from a complete population.

To address this, researchers in Denmark used national healthcare data to look for links between mRNA vaccination and a hospital diagnosis of myocarditis or myopericarditis, increased blood troponin levels (a measure of myocardial damage), and a hospital stay lasting more than 24 hours.

Their analyses included nearly five million Danish residents aged 12 years or older who received either the Pfizer/BioNTech or Moderna vaccines.

Participants were monitored from Oct 1, 2020, to Oct 5, 2021.

A range of potentially influential factors were also 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- to 39-years-old and 196 (73%) were male.

Overall, the results show a strong association between vaccination with Moderna and myocarditis or myopericarditis, while vaccination with Pfizer/BioNTech was only associated with an increased rate of myocarditis or myopericarditis among women.

The rate of myocarditis or myopericarditis was higher for Moderna vaccination than for Pfizer/BioNTech vaccination.

Nevertheless, the absolute number of events after either 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.

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 youngest age group (12–17-year-olds), it was only one per 100,000, within 28 days of receiving the Pfizer/BioNTech vaccine.

Of 498,814 individuals vaccinated with Moderna, 21 developed myocarditis or myopericarditis within 28 days of vaccination (an absolute rate of 4.2 per 100,000), compared with unvaccinated individuals.

Among 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 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, so can’t establish cause, and the researchers point to some potential sources of bias, such as increased public awareness of potential side effects of vaccines, that 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.

As such, the researchers conclude that mRNA 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.

The benefits of vaccination should be taken into account when interpreting these findings, they add, and larger multinational studies are needed to further investigate the risks of myocarditis or myopericarditis after vaccination within smaller groups.