A large study finds that a third dose of the Moderna or Pfizer COVID-19 vaccine poses no more risk of heart infection than a second dose. The results can help allay concerns about increased risks with each additional dose.

Cases were rare. Most appeared within seven days of vaccination and patients recovered, finds the study, which was presented Saturday at the American Heart Association Scientific Sessions. The work is preliminary until the full results are published in a peer-reviewed journal.

Researchers examined the health records of millions of Southern Californians to see if the third doses of the vaccines, which use mRNA technology to activate the body’s defenses against the coronavirus, were linked to myocarditis, an inflammation of the heart muscle.

“It’s an important question because with additional doses of COVID-19 mRNA vaccines being recommended, it’s essential to monitor its safety,” said Dr. Mingsom Lee, lead study author and a cardiologist at Kaiser Permanente Los Angeles Medical Center.
Myocarditis is a rare and potentially serious side effect of the COVID-19 vaccine, often after a second dose than the first, according to the Centers for Disease Control and Prevention.
Lee and colleagues examined the records of adults who were part of a Kaiser Permanente Southern California health plan and received at least one dose of the vaccine between December 14, 2020 and February 18, 2022, before updated “bivalent” booster formulations became available. The average age of the subjects in the study was 47. About 54% of the subjects in the study were women. 39% were Hispanic. 30% were white; 13% were Asian; and 7% were black.
Researchers looked at medical records for people hospitalized with myocarditis within 21 days of receiving the Pfizer or Moderna vaccines.
Of the more than 3 million members who received the first dose, six were hospitalized with myocarditis. Of the 2.9 million people who received a second dose, 26 developed myocarditis that required hospitalization — 21 cases that occurred within seven days. Of the 1.4 million who received a third dose, nine were hospitalized with myocarditis, and seven of those developed within a week.
Compared with what would be expected in the same population during the one-year period before the epidemic, the incidence of myocarditis was four times greater after the second dose and 2.6 times greater after the third dose.
But overall, myocarditis was rare, most cases were mild, and symptoms resolved without extensive intervention, said Lee, also a medical investigator with the Division of Research and Evaluation at Kaiser Permanente Southern California.
Eight of the nine cases of post-dose myocarditis were in men, and five were in people aged 18 to 40 years. Previous studies have shown an increased risk of developing myocarditis after vaccination in adolescents and young adults.
Such risks should be kept in context, Lee said, given that other studies have shown that the risk of developing myocarditis is much higher after infection with COVID-19 than after vaccination.
Epidemiologist Howard Siso, MD, associate professor of medicine in the Department of Preventive Medicine at Brigham and Women’s Hospital in Boston, agrees. “This should not change one’s decision about whether or not to vaccinate or booster,” he said. “The benefits still far outweigh any potential small risks like this.”
In addition to myocarditis, COVID-19 increases the risk of other complications, and has killed more than 1 million people in the United States.
The CDC recommends that people 5 years of age and older receive a bivalent booster dose if it has been at least two months since the last dose of the COVID-19 vaccine.
Sisso, who was not involved in the new study, said the sheer amount of EHR benefit was impressive. He said that while the results were not surprising, they are “important and suggestive findings that must be replicated” and followed up.
He said that even a large study faces challenges in raising accurate information such as a possible link to myocarditis, and this type of research can only show the association, not cause and effect.
He said more research is needed on the possible biological explanations behind any link. To that end, he would be interested to see more data and analysis comparing the people in this study who developed myocarditis with those who did not.
The researchers noted some limitations of their work, including the potential for some cases of myocarditis to be lost if people did not seek hospital care.

Lee said more research will be needed as vaccine and coronavirus recommendations continue to evolve. Some people take their fifth dose already.
“Safety monitoring associated with additional vaccine doses will be important,” she said.
Editor’s note: Due to the rapidly developing events surrounding the coronavirus, the facts and advice presented in this story may have changed since its publication. Visit Heart.org for the latest coverage, and check with the Centers for Disease Control and Prevention and local health officials for the latest guidelines.
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