Your Choice of COVID Vaccine Can Increase Your Risk of Myocarditis

The study found that Moderna had greater rates of heart inflammation than Pfizer, although the overall risk remained extremely low.

In comparison to the Pfizer BioNTech COVID-19 vaccine, the Moderna Spikevax COVID-19 vaccine has a two- to three-fold greater incidence of myocarditis, pericarditis, or myopericarditis following a second dose; nonetheless, overall cases of heart inflammation with either vaccine are very rare. Males under 40 who got the Moderna vaccine had the greatest incidence of myocarditis, according to the research, which, according to the scientists, may have consequences for choosing certain vaccines for particular populations.

The findings were recently published in the Journal of the American College of Cardiology.

Pfizer BioNTech (BNT162b2) and Moderna Spikevax (mRNA-1273) are the two mRNA COVID-19 vaccines that have been given approval for usage, and as of March 20, 2022, more than 52 million doses of Pfizer and 22 million doses of Moderna have been given in Canada, where this study was conducted. Clinical trials have shown that the vaccines are safe, and monitoring of vaccinated people has shown that side effects are minor and disappear on their own. However, both vaccines have been associated with some rare but serious side effects, most notably myocarditis (inflammation of the heart).

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While several studies have been done on each vaccine, few have been done to directly compare the two mRNA vaccines’ safety. The purpose of this research was to compare the risks of myocarditis, pericarditis, and myopericarditis associated with 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 shots were administered outside of British Columbia or had a history of myocarditis or pericarditis within one year prior to the 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.

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

“Few population-based analyses have been conducted to directly compare the safety of the two mRNA COVID-19 vaccines, which differ in important ways that could impact safety,” said Naveed Janjua, MBBS, DrPH, lead author of the study and an epidemiologist and the executive director of Data and Analytic Services at the British Columbia Centre for Disease Control. “Our findings have implications for strategizing the rollout of mRNA vaccines, which should also consider the self-limiting and mild nature of most myocarditis events, benefits provided by vaccination, higher effectiveness of the Moderna vaccine against infection and hospitalization [found in prior studies], and the apparent higher risk of myocarditis following COVID-19 infection than with mRNA vaccination.”

Limitations of the study include that it was observational, which limits the ability to determine causality between vaccination and myocarditis or pericarditis. However, temporality was ensured in the study design to limit the time studied between vaccine dose and myocarditis/pericarditis diagnosis. Also, the study relied on hospital and emergency department visit data and may have missed some less severe cases.

In a related editorial comment, Guy Witberg, MD, MPH, a cardiologist at Rabin Medical Center in Petah-Tikva, Israel, wrote the study is reassuring for vaccine safety since it provides further data that myocarditis is a very rare adverse event after both vaccines, and it is an important step toward a personalized approach to administering COVID-19 vaccines.

“[The study] should help put to rest ‘vaccine hesitancy’ due to concerns over cardiac adverse events,” Witberg said. “This is one of only a few direct comparisons of the two widely adopted mRNA vaccines, and its results have practical policy implications: for a substantial segment of the population suffering from cardiovascular disease…these data give a strong argument to preferentially use the BNT162b2 [Pfizer] vaccine over mRNA-1273 [Moderna].”

References: “Comparative Risk of Myocarditis/Pericarditis Following Second Doses of BNT162b2 and mRNA-1273 Coronavirus Vaccines” by Zaeema Naveed, Julia Li, James Wilton, Michelle Spencer, Monika Naus, Héctor A. Velásquez García, Jeffrey C. Kwong, Caren Rose, Michael Otterstatter and Naveed Z. Janjua, 7 November 2022, Journal of the American College of Cardiology.
DOI: 10.1016/j.jacc.2022.08.799

“A Tale of 2 mRNA Vaccines” by Guy Witberg and Ilan Richter, 7 November 2022, Journal of the American College of Cardiology.
DOI: 10.1016/j.jacc.2022.09.010

The study was funded by the British Columbia Centre for Disease Control and the Canadian Immunization Research Network (CIRN) through a grant from the Public Health Agency of Canada and the Canadian Institutes of Health Research. This project was also supported by funding from the Public Health Agency of Canada through the Vaccine Surveillance Reference Group and the COVID-19 Immunity Task Force.