A new study estimated that hundreds of cases of heart inflammation occur in every million second doses of Moderna's COVID-19 vaccine in the highest-risk group.
Some 269.5 cases of myocarditis, a form of heart inflammation, were estimated for men aged 18 to 29, researchers concluded.
Myocarditis is a serious condition that can lead to death.
Researchers drew the numbers from a public health surveillance system established by authorities in British Columbia in Canada.
In the population-based cohort study, researchers looked for cases of myocarditis, pericarditis, and myopericarditis shortly after receipt of a Moderna or Pfizer COVID-19 vaccine, both of which utilize messenger RNA technology. Pericarditis is inflammation of tissue near the heart and myopericarditis is when both myocarditis and pericarditis occur.
Researchers excluded people who didn't receive the two-shot primary series, who received one or more doses outside of British Columbia, and who experienced myocarditis within one year before vaccination.
Out of the remaining 3.1 million people, 59 developed myocarditis and 41 developed pericarditis between seven and 21 days after vaccination.
Researchers took the numbers and translated them to rates per million by using modeling.
The highest rates of myocarditis were among young males, who have long been found to be at the highest risk of post-vaccination heart inflammation.
Moderna's rate was 269.5 per million second doses among males aged 18 to 29. Pfizer's rate was 58 per million second doses among males in the age group.
The rates dropped in older age groups. Rates for males aged 30 to 39, for instance, were 54 per million (Moderna) and 11.3 per million (Pfizer).
Among women aged 18 to 29, the rates were 27.4 for Moderna and 4.8 for Pfizer. Among women aged 30 to 39, the rates were 27.8 for Moderna and 11.4 for Pfizer.
Background rates, or rates in the general population, are just 2 per million in people aged 18 to 39, meaning vaccination increases the likelihood of incidence.
Compared with Pfizer's shot, the Moderna vaccine was linked with more than twofold higher odds of developing the inflammation conditions, researchers said.
Pfizer and Moderna didn't respond to requests for comment by press time.
"Our findings may have policy implications regarding the choice of vaccine offered," the researchers said.
Limitations included the study being observational, with researchers saying they couldn't tell whether the inflammation cases were related to vaccination.