Risk of rare heart inflammation may be higher after Moderna than Pfizer COVID vaccine

Stethoscope on man's chest
Stethoscope on man's chest

wutwhanfoto / iStock

Myocarditis and pericarditis are rare after mRNA COVID-19 vaccination, but rates of the inflammatory heart conditions were twofold to threefold higher after receipt of the second dose of the Moderna vaccine than after the Pfizer/BioNTech formulation, suggests a head-to-head comparison in Canadian adults.

The findings of the observational, population-based analysis were published yesterday in the Journal of the American College of Cardiology.

Researchers from the British Columbia Centre for Disease Control in Vancouver led the study on the diagnosis of myocarditis, pericarditis, or myopericarditis during a hospitalization or emergency department visit within 21 days after receipt of the second mRNA COVID-19 vaccine dose from Jan 1 to Sep 9, 2021. During that period, more than 870,000 Moderna and 2.2 million Pfizer second doses were administered in British Columbia.

Myocarditis is inflammation of the heart muscle, pericarditis is inflammation of the membrane surrounding the heart, and myopericarditis is an extension of pericardial inflammation into the heart muscle.

Risk after COVID infection higher

Rates of myocarditis (31 cases; 35.6 per million second doses) and pericarditis (20; 22.9 per million) were higher after the Moderna vaccine than after Pfizer (28; 12.6 per million and 21; 9.4 per million, respectively). For comparison, rates of myocarditis in the general population during the same period were 2.0 per million in vaccinees 18 to 39 years old and 2.2 per million in older adults.

Relative to the Pfizer vaccine, Moderna was tied to significantly higher chances of myocarditis (adjusted odds ratio [aOR], 2.78; 95% CI, 1.67 to 4.62), pericarditis (aOR, 2.42; 95% CI, 1.31 to 4.46), and myopericarditis (aOR, 2.63; 95% CI, 1.76 to 3.93). The link between Moderna and myocarditis was strongest for men (aOR, 3.21; 95% CI, 1.77 to 5.83) and the younger age-group (aOR for 18 to 39 years, 5.09; 95% CI, 2.68 to 9.66).

A person choosing an mRNA vaccine should "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," lead author Naveed Janjua, MBBS, PhD, said in an American College of Cardiology news release.

In a related commentary, Guy Witberg, MD, MPH, and Ilan Richter, MD, MPH, both of Rabin Medical Center in Petah-Tikva, Israel, said the study provides further evidence that heart inflammation is rare after both vaccines.

It "should help put to rest 'vaccine hesitancy' due to concerns over cardiac adverse events," they wrote. "Its results have practical policy implications: for a substantial segment of the population suffering from cardiovascular disease, especially those with left ventricular dysfunction, in whom minimizing risk of myocardial insult is crucial, these data give a strong argument to preferentially use the BNT162b2 [Pfizer] vaccine over mRNA-1273 [Moderna]."

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