Another Study Finds Heart Inflammation Higher After Moderna Vaccination Versus Pfizer

A nurse prepares a COVID-19 vaccine in Toronto on March 23, 2021. (Cole Burston/Getty Images)

Cases of heart inflammation after COVID-19 vaccination were more common among Moderna recipients than those who received Pfizer’s shot, according to a new study.

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.

Latest to Find Moderna Higher

Research dating back to mid-2021 shows that the incidence of heart inflammation is higher following a Moderna second dose for young males when compared to a Pfizer second dose.

Both vaccines are recommended as two-dose primary series.

Dr. Anish Koka, a cardiologist based in the United States, said on Twitter that the new study highlights the lack of action by the U.S. Centers for Disease Control and Prevention (CDC), which continues to recommend that young males receive either vaccine.

“The rates of Moderna are really much higher for dose 2 in young men,” Dr. Walid Gellad, a professor of medicine at the University of Pittsburgh, said. “I remain perplexed why US never acted on this information, which has been known for a year.”

Some other countries have suspended administration of Moderna’s vaccine—or both vaccines—for young people based on the vaccine side effects and the fact that healthy youth face little risk from COVID-19.

The CDC has also detected more cases of myocarditis (pdf) after receipt of a Moderna second dose in the highest-risk populations, using surveillance data.

U.S. authorities added myocarditis as a possible side effect for both vaccines in 2021, but have not changed their recommendations, which call for virtually all people to receive not only a primary series, but at least one booster shot.

U.S. authorities have said the benefits of the vaccines—primarily protection against severe illness—outweighs the risks.

The Canadian researchers said as much, though their only citation was to a non-peer-reviewed CDC paper from June 2021.

Other studies since then have concluded that the risks outweigh the benefits for one or more populations, particularly young males. The calculus has tilted because of the growing evidence of side effects like myocarditis and the worse performance of the vaccines against the Omicron virus variant and its subvariants, some experts say.

More on New Paper

The Canadian researchers analyzed information from a British Columbia surveillance platform that has data such as laboratory tests and hospital admissions. They examined data from Dec. 15, 2020, to March 10, 2022.

They found that 105 males and 36 females experienced myocarditis and went to a hospital or emergency room within 21 days of a shot.

Approximately 60 percent of the cases happened after a Pfizer jab, but the overall dataset included a higher level of Pfizer administration than Moderna administration.

Researchers calculated an overall rate of 1.37 cases of myocarditis per 100,000 mRNA vaccine doses, above the expected rate of 0.39 cases per 100,000 population. The expected rate was drawn from the incidence of myocarditis in the general population from before the pandemic.

The rates were far higher after a second dose and among young males.

For males aged 18 to 29 after a second dose, the rate was 23 per 100,000 after a Moderna shot and 5.8 per 100,000 after a Pfizer shot.

For males aged 30 to 39 after a second dose, the rate was 7 per 100,000 after a Moderna shot and 1.3 per 100,000 after a Pfizer shot.

For males aged 40 to 49 after a second dose, the rate was 1.3 per 100,000 after a Moderna shot and 1.1 per 100,000 after a Pfizer shot.

Researchers calculated the rate within 21 days as 6.7 per 100,000 second doses of Pfizer’s vaccine for males aged 12 to 17, but were unable to calculate the figure for that age group for Moderna’s vaccine because the vaccine was not available in Canada for that group during the time period that was analyzed.

Depending on the dose, the rates were higher for Pfizer’s vaccine.

While the rates were higher after Moderna’s second shot for both males and females aged 18 to 39, for instance, the rates were higher after Pfizer’s first shot for males and females between the ages of 29 and 40.

Rates were also higher after receipt of a third Pfizer dose than a third Moderna dose for some of the age groups, including males aged 18 to 29. That was also the case with the CDC surveillance data. A third dose, or a booster, has become widely accepted because the vaccines perform worse against newer variants and the protection they confer rapidly wanes.