COVID-19: Canadian study finds heart risks like myocarditis higher with Moderna


The risk of developing heart inflammation is two to three times higher with the Moderna COVID-19 vaccine when compared to Pfizer, according to a new Canadian study.

Published in the Journal of the American College of Cardiology, the study found that although cases were rare, men under the age of 40 were the most at-risk of developing heart complications like myocarditis and pericarditis, which usually appeared within 21 days of a second vaccination dose.

Myocarditis refers to inflammation of the heart muscle, while pericarditis is inflammation of the outside lining of the heart.

“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,” the study’s lead author, British Columbia Centre for Disease Control epidemiologist Dr. Naveed Janjua, said in a news release.

The study tracked people 18 years or older who received a second dose of either the Pfizer BioNTech or Moderna Spikevax COVID-19 vaccines in British Columbia between Jan. 1, 2021 and Sept. 9, 2021. In total, more than 2.2 million second Pfizer doses were administered in B.C. during that period, plus 870,000 Moderna doses.

Within 21 days of a second dose, there were 59 myocarditis cases (28 Pfizer and 31 Moderna) and 41 pericarditis cases (21 Pfizer and 20 Moderna). Researchers then calculated the rate of heart inflammation, and found there were 35.6 cases per million for Moderna and 12.6 per million for Pfizer – a nearly threefold difference. Rates for myocarditis were higher with the Moderna vaccine in both males and females between the ages of 18 and 39, with even higher rates observed in males 18 to 29. To compare, rates of myocarditis in the general population in 2018 were 2.01 to 2.2 per million people, according to the study.

In addition to being an uncommon but troubling side effect to mRNA vaccines, myocarditis has also been linked to viral infections like COVID-19 and influenza. A recent study found that the risk of developing myocarditis is seven times higher with a COVID-19 infection than a vaccine dose. From December 2021 to April 2022, Canadian soccer star Alphonso Davies was sidelined with myocarditis following a bout of COVID-19.

Dr. Janjua is also the executive director of data and analytic services at the British Columbia Centre for Disease Control and a clinical professor in the University of British Columbia’s population and public health department. He says the finding should help direct which populations receive which vaccine to maximize benefits while minimizing adverse affects.

“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,” Janjua explained.

One limit to the study is that it was observational, which means it did not prove that vaccination caused myocarditis or pericarditis. Also, by relying on hospital and emergency department data, the study may have missed some less severe cases.

In an editorial comment on the study, Israel-based cardiologists Dr. Guy Witberg and Dr. Ilan Richter wrote that the results are “reassuring in terms of vaccine safety and should help put to rest ‘vaccine hesitancy’ caused by concerns over cardiac adverse events; such a conclusion leans not only on the proven efficacy of the vaccines, but also on data showing that COVID-19 infection is associated with a much higher risk for myocarditis.”

Over 93 million COVID-19 vaccine doses have been administered in Canada, including nearly 62 million Pfizer shots and more than 28 million of Moderna. Both companies have committed to studying the long-term effects of vaccine-related heart problems.

A January 2022 study found the rate of myocarditis in the U.S. to be 52.4 and 56.3 per million for Pfizer and Moderna in young men aged 18 to 24. In June, the U.S. Centers for Disease Control and Prevention cautioned that the Moderna vaccine may pose a higher risk of myocarditis in some age groups.

According to the Public Health Agency of Canada, myocarditis and pericarditis remain rare complications. The agency urges anyone experiencing chest pain or pressure, an irregular heartbeat or shortness of breath after vaccination to seek medical attention. It says cases appear to be occur most amongst adolescents and young adults, males, following a second dose, and within seven days of vaccination.

“In most cases, someone who has had myocarditis or pericarditis after an mRNA vaccine should defer receiving another dose,” the agency states. “The rates of these conditions after the booster dose with an mRNA vaccine appear to be somewhat lower than after the second dose.”


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