New study provides reassuring data on rare heart condition after COVID-19 vaccination

A new study has provided reassuring data on the rare heart condition that some people developed after receiving a COVID-19 vaccine, finding that clinical outcomes of myocarditis or myopericarditis were predominantly mild and generally similar between vaccinated and unvaccinated individuals.

The study, entitled “SARS-CoV-2 vaccination and myocarditis or myopericarditis: population based cohort study,” was published in the British Medical Journal on Thursday.

Myocarditis is the inflammation of the heart muscle and myopericarditis is the inflammation of the outer lining of the heart. Both are serious but rare conditions usually trigged by a viral, bacterial or fungal infection.

Recent reports and studies have indicated an increased risk of heart inflammation after receiving a mRNA vaccine, especially after a second dose, leading to health regulatory bodies adding updates to the labels of both Moderna and Pfizer’s vaccines, or taking them off the table for certain populations.

However, no study has investigated the association of mRNA COVID-19 vaccinations and myocarditis or myopericarditis using information from a complete population, according to a release.

In a first-of-its-kind study, Danish researchers used national health-care data to look for links between mRNA vaccination and a hospital diagnosis of both rare heart conditions, increased blood troponin levels — a measure of heart damage — and a hospital stay lasting more than 24 hours.

The study included nearly five million Danish residents aged 12 years and older who received either the Pfizer or Moderna vaccine, monitoring participants from Oct.1, 2020 to Oct. 5, 2021. Researchers took into account mitigating factors such as age, sex, vaccine priority grouping and any underlying health conditions.

Researchers did not include participants who received two different COVID-19 vaccines nor those who had a positive COVID-19 test to avoid associating outcomes from SARS-CoV-2 infection with vaccination.

In the follow-up period, researchers logged 269 participants who developed myocarditis or myopericarditis, 108 (or 40 per cent) of whom were between the ages of 12 to 39 years-old and 196 (73 per cent) were male.

Overall results showed a strong association with Moderna and myocarditis or myopericarditis, defined “as the combined outcome of a hospital diagnosis of either heart condition, increased troponin levels and a hospital stay longer than 24 hours.”

The Pfizer vaccine was only associated with an increased rate of the rare heart conditions amongst women.

Overall, individuals vaccinated with Pfizer had a “non-significantly increased rate” of myocarditis or myopericarditis in the 28 days after vaccination compared with unvaccinated follow-up after adjustment for age, sex, vaccine priority group, season, and clinical comorbidities, according to the study. Among participants aged 12 to 39 years old, the study found a “non-significantly increased rate” in the 28 days after vaccination compared with unvaccinated follow-up.

The rate of myocarditis or myopericarditis was higher for Moderna vaccination than with Pfizer, but researchers observed “no readmissions, diagnoses of heart failure or deaths among people with myocarditis or myopericarditis occurring within 28 days” of a Moderna vaccination.

However, the “absolute number of events” after either vaccine was low, even in younger age groups, and cases were predominantly mild – providing further evidence to support the overall safety of mRNA vaccines for COVID-19.

For example, the release states, out of 3.4 million individuals vaccinated with Pfizer, 48 developed myocarditis or myopericarditis within 28 days of vaccination, a rate of 1.4 per 100,000.

Among women, the rate of developing the rare heart conditions with Pfizer was 1.3 per 100,000 compared 1.5 per 100,000 in men.

Among the 12 to 39 age group, the rate was 1.6 per 100,000 and in the youngest cohort of 12 to 17 year-olds, it was only one per 100,000 within 28 days of receiving a Pfizer vaccine.

Out of 498,814 individuals vaccinated with Moderna, 21 developed myocarditis or myopericarditis within 28 days of vaccination, a rate of 4.2 per 100,000.

For Moderna, the rate of developing the rare heart conditions for women was two per 100,000 compared to 6.3 per 100,000 in men. Among the 12-to-39 age group, the rate was 5.7 per 100,000 within 28 days of receiving a Moderna vaccine.

Both vaccines were associated with approximately a 50 per cent reduced risk of cardiac arrest or death, the most severe outcome from myocarditis or myopericarditis, the researchers said.

In contrast, researchers found a 14-fold increased risk of cardiac arrest or death 28 days after a positive COVID-19 test, compared with uninfected individuals.

The researchers are calling for larger multinational studies on the subject to specify risks of the rare heart conditions within smaller subgroups and the risk of myocarditis or myopericarditis after COVID-19 infection versus vaccination. 

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