One of the largest studies to date on myocarditis after COVID-19 vaccination confirms an increased risk with both the Pfizer and Moderna vaccines in young men and shows that the risk is higher with the Moderna than with the Pfizer vaccine.
The study also suggests for the first time that in young men 16 to 24 years of age, the risk for myocarditis after vaccination with either the Pfizer or Moderna vaccine is higher than the risk for myocarditis after COVID-19 infection.
The population-based study involved data on 23.1 million residents across four Scandinavian countries – Denmark, Finland, Norway, and Sweden – 74% of whom had received two vaccine doses and 7% of whom had received one dose.
By linking data from high-quality nationwide health registers on COVID-19 vaccination, infection rates, and myocarditis diagnoses, the researchers were able to evaluate the risk for myocarditis by vaccine product, vaccination dose number, sex, and age.
The study was published online in JAMA Cardiology.
The results confirm that the risk for myocarditis after COVID-19 mRNA vaccines is highest in young men 16 to 24 years of age after the second dose.
For men in this age group who received two doses of the same vaccine, data were compatible, with between four and seven excess myocarditis events in 28 days per 100,000 individuals after the second dose of the Pfizer vaccine, and between nine and 28 per 100,000 individuals after the second dose of the Moderna vaccine.
“This is one of the largest studies on this topic to date. The first population studies were in Israel, with 5 million individuals, and looked at just the Pfizer vaccine. We have data on 23 million people from Scandinavia that include both the Pfizer and Moderna vaccines,” senior author Rickard Ljung, MD, Swedish Medical Products Agency, told this news organization.
“We show a clearly higher risk of myocarditis after the Moderna vaccine than after the Pfizer vaccine. This has been suggested before, but our data confirm definitively that the Moderna vaccine has a higher risk of myocarditis than the Pfizer vaccine,” he added.
“In the group at highest risk of myocarditis after COVID vaccination – young men aged 16 to 24 – the Pfizer vaccine shows a five times higher risk of myocarditis versus the unvaccinated cohort, while the Moderna vaccine shows a 15 times higher risk,” Dr. Ljung noted.
After seeing these data, the Swedish regulatory authority is no longer recommending use of the Moderna vaccine for people younger than 30 years, Dr. Ljung said. Similar recommendations have been made in Norway and Finland.
The researchers report that their finding of a higher risk for myocarditis after the Moderna vaccine than after the Pfizer vaccine in young men is in line with data from the Canada, France, the United Kingdom, and the United States. But they point out that, compared with previous studies, the current study had the advantage of data analyzed according to a common protocol from four different countries and that showed similar directions of associations, despite considerable differences in previous COVID-19 infection levels and lockdown policies.
Risk higher with vaccination than infection?
For what is believed to be the first time, the Scandinavian data also suggest a higher risk for myocarditis after COVID-19 vaccination with both the Pfizer and Moderna vaccines than after COVID-19 infection in young men 16 to 24 years.
Although previous studies have shown that males in this age group have the highest risk for myocarditis after vaccination, it has always been suggested that the risk after vaccination is lower than the risk after infection. The Scandinavian data suggest otherwise for this age group.
Dr. Ljung explained that the myocarditis risk after COVID infection is very hard to study.
“It is highly dependent on the testing strategy,” he said. “For example, in the first half of 2020, the only people being tested were those admitted to hospital, so studies would have included the sickest patients and would therefore likely have found a higher rate of myocarditis. But this current Scandinavian dataset only included individuals with a positive COVID test after August 2020, reflecting a broader range of people.”
The researchers found an excess rate of myocarditis of 3.26 per 100,000 individuals within 28 days of a positive COVID-19 test among all males, and 1.37 per 100,000 individuals among males 16 to 24 years of age.
“We show that the risk of myocarditis after COVID infection is lower in younger people and higher in older people, but the opposite is true after COVID vaccination, where the risk of myocarditis is higher in younger people and lower in older people,” Dr. Ljung said.
The study was not able to look at severity of myocarditis but did record length of hospital stay, which was similar in patients who developed myocarditis after vaccination and those in the unvaccinated cohort (4 to 5 days). Deaths were rare, with no deaths in people younger than 40 years.
“I think we can say that in people aged over 40, the risk of myocarditis is greater with infection than with vaccination, but in those under 40, it is not so clear. And our data suggest that for young men aged 16 to 24 years, the risk of myocarditis after COVID vaccination with either the Pfizer or Moderna vaccine is higher than after COVID infection,” Dr. Ljung commented.
Although the Swedish regulatory agency has already stopped recommending use of Moderna vaccine in those younger than 30 years on the basis of these data, Dr. Ljung was reluctant to make any recommendations regarding the use of the Pfizer vaccine in young males, saying it was up to individual public-health agencies to makes these decisions.
But he pointed out that the current study only looked at myocarditis, and COVID infection can result in many other complications that can lead to hospitalization and death, which needs to be taken into account when assessing the risk and benefit of vaccination.
Dr. Ljung noted that the current data only applied to the first two doses of the vaccines; data after booster injections have not been included, although the researchers are looking at that now.