A new study published in the Journal of American College of Cardiology found the risk of myocarditis and pericarditis (heart inflammation) following receipt of an mRNA COVID-19 biological is two to three times higher among individuals who received the Spikevax shot developed by Moderna/NIAID versus the Comirnaty shot developed by Pfizer/BioNTech.1
The study was conducted in British Columbia, Canada where the Moderna and Pfizer COVID shots are being widely distributed.2 The U.S. Food and Drug Administration (FDA) awarded Moderna and Pfizer an Emergency Use Authorization (EUA) to distribute the mRNA biological products in the United States in December 2020. According to the Bloomberg Vaccine Tracker, by Oct. 5, 2022 about 613 million doses of Spikevax and Comirnaty had been administered in the U.S.3
Ingredient Differences Between the Two COVID mRNA Shots
The main difference between the Moderna and Pfizer mRNA products is that an adult dose of Moderna’s product contains 100 mcg of mRNA, while an adult dose of Pfizer’s product contains 30 mcg of mRNA.4 There are also differences in the ingredients contained in both shots, including the type of lipids used to transport mRNA to cells.5 Both Spikevax and Comirnaty inject synthetic mRNA enveloped in lipid nanoparticles into the body, where the mRNA gives genetic instructions to cells to produce the SARS-CoV-2 spike protein.6
Post-marketing evaluations have linked myocarditis and pericarditis (heart inflammation) to mRNA COVID shots. However, there are very few scientific analyses that have directly compared the safety of the two mRNA COVID shots currently on the market.7
As of Mar. 18, 2022, there were 1,933 post-vaccination cases of myocarditis/pericarditis reported in Canada. Other studies have found that the rate of myocarditis/pericarditis following mRNA COVID vaccination is higher than initially expected, especially in younger men following a second dose of an mRNA COVID shot.
While the development of myocarditis or pericarditis has been observed after both shots, the new study reveals that there appears to be a higher risk following a second dose of Moderna’s Spikevax.8
Rates of Myocarditis and Pericarditis Higher for Young Men After mRNA COVID Shots, Regardless of Manufacturer
The large population-based cohort study analyzed people who received two doses of the mRNA COVID shots and found that post-vaccination myocarditis and pericarditis rates were higher for men and in younger age groups for both biologics, with the highest rate observed in men aged 18 to 39 years following a second dose of Moderna’s Spikevax.9 10
Canadian data showed that the rates of myocarditis after a second dose of the shots were much higher for Spikevax than Comirnaty among individuals 18-39 years of age, with a rate per million doses of 195.5 versus 44.3 among individuals 18 to 24 years old.11
The study authors concluded that the “findings may have policy implications regarding the choice of vaccine offered.” The study’s lead author, epidemiologist Dr. Naveed Zafar Janjua, told UPI that the new data:
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.12
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1 Naveed Z et al. Comparative Risk of Myocarditis/Pericarditis Following Second Doses of BNT162b2 and mRNA-1273 Coronavirus Vaccines. Journal of American College of Cardiology 2022; 80(20): 1900–1908.
3 Bloomberg. More Than 12.7 Billion Shots Given: COVID-19 Tracker. Oct. 6, 2022.
4 Schuster-Bruce C. Pfizer picked a COVID-19 vaccine dose far lower than Moderna’s to minimize side effects, its top scientist says. Business Insider Sept. 8, 2021.
5 Ravell JC, Hanhan ZG. A Simple Breakdown of the Ingredients in the COVID Vaccines. Hackensack Meridian Health Jan. 11, 2021.
6 Fox A, Bond C. You don’t need to worry about COVID vaccines being ‘unnatural’ or ‘synthetic.’ The Conversation Sept. 1, 2021.
7 Naveed Z et al. Comparative Risk of Myocarditis/Pericarditis Following Second Doses of BNT162b2 and mRNA-1273 Coronavirus Vaccines. Journal of American College of Cardiology 2022; 80(20): 1900–1908.
12 Packer-Tursman J. Study: Myocarditis risk 2 to 3 times higher from Moderna than Pfizer COVID-19 vaccine.UPI Nov. 7, 2022.