Canadian researchers analyzed a database and identified 141 cases of myocarditis, a form of heart inflammation, within 21 days of a dose of the Pfizer or Moderna vaccine, both of which utilize messenger RNA (mRNA) technology.
That was compared with an expected number of just 20 cases.
Cases were much higher for young males, as previous studies have found, but were elevated even higher following receipt of a second dose of the Moderna vaccine compared with a second dose of the Pfizer shot.
The incidence, though, was higher after receipt of a third dose of the Pfizer vaccine.
“In this population-based cohort study, observed rates of hospital admissions or emergency department visits for myocarditis after mRNA vaccination for SARS-CoV-2 were higher than expected based on historical background rates, particularly after the second dose, among those who received the mRNA-1273 (Moderna) vaccine, among males and among younger patients (18–29 yr),” Dr. Zaeema Naveed and other researchers with the University of British Columbia and British Columbia Centre for Disease Control wrote.
The paper was published in the Canadian Medical Association Journal on Nov. 21.
Moderna and Pfizer did not respond to requests for comment.