Heart inflammation after a COVID-19 vaccine is very rare but epidemiology exists to vreate exploratory between products and outcomes and a recent analysis showed incidence of myocarditis, pericarditis or myopericarditis is two- to threefold higher after a second dose of the Moderna Spikevax COVID-19 vaccine compared to the Pfizer BioNTech COVID-19 vaccine.

That's not a reason to avoid a vaccine, any more than you should stop eating salad if it has a regular pesticide rather than an organic one, but it may mean that it's beneficial to choose specific vaccines for certain populations.

 Pfizer BioNTech (BNT162b2) and Moderna Spikevax (mRNA-1273) are both mRNA COVID-19 vaccines and by March 20, 2022 over 52 million doses of Pfizer and 22 million doses of Moderna were administered in Canada. People in the data pool 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.

Like all epidemiology, the limitations are important and why they are exploratory. It was only observational, there is no way to show causality between vaccination and myocarditis or pericarditis. The authors also relied on hospital and emergency department visit data and may have missed some less severe cases.  

According to the authors, the findings support recommending certain populations receive certain vaccines to maximize benefits and minimize adverse events.