Incidence of myocarditis, pericarditis or myopericarditis is two- to threefold higher after a second dose of the Moderna Spikevax COVID-19 vaccine when compared to the Pfizer BioNTech COVID-19 vaccine; however, overall cases of heart inflammation with either vaccine are very rare, according to a study in the Journal of the American College of Cardiology.
The study showed males younger than 40 years old who received the Moderna vaccine were shown to have the highest rates of myocarditis, which according to the authors, may have implications for choosing specific vaccines for certain populations.
While there have been many studies on either vaccine, few studies have been conducted to directly compare the safety of the two mRNA vaccines. Researchers in this study sought to compare the risk of myocarditis, pericarditis and myopericarditis between the Pfizer and Moderna COVID-19 vaccines.
People in the study were 18 years old or older and had received two primary doses of either Pfizer or Moderna vaccine in British Columbia, Canada, with the second dose between Jan. 1, 2021 and Sept. 9, 2021. Individuals whose first or second shot were administered outside of British Columbia or had a history of myocarditis or pericarditis within one year prior to second dose were excluded.
In all, more than 2.2 million second Pfizer doses were given and more than 870,000 Moderna doses. Within 21 days of the second dose, there were a total of 59 myocarditis cases (21 Pfizer and 31 Moderna) and 41 pericarditis cases (21 Pfizer and 20 Moderna). Researchers also looked at rates per million doses and the rate was 35.6 cases per million for Moderna and 12.6 per million for Pfizer—an almost threefold increase after Moderna shots vs. Pfizer. Comparatively, rates of myocarditis in the general population in 2018, were 2.01 per million in people under age 40 and 2.2 per million in people over age 40.
Rates of myocarditis and pericarditis were higher with the Moderna vaccine in both males and females between ages 18 and 39, with the highest per million rates in males ages 18-29 after a second dose of Moderna.