NEW YORK (NYTIMES) - A second dose of the coronavirus vaccines made by Pfizer-BioNTech and Moderna can nearly triple the chances of a rare heart condition in young men, according to a large new study published on Monday (Oct 4). But the absolute risk of the condition, called myocarditis, remains extremely low.
The study found 5.8 cases per million second doses in men, with an average age of 25 years. The risk after the first dose was much less, at 0.8 cases per million, not more than normally would be seen in that age group. The findings were published in the journal JAMA Internal Medicine.
The results may be underestimates. The researchers tracked the medical status of vaccinated individuals for only a short time, and may have missed people whose heart problems were not severe enough to require hospitalisation.
Concerns about myocarditis, an inflammation of the heart muscle, have been the subject of intense discussion among advisers to the Centres for Disease Control and Prevention, and may have led the Food and Drug Administration to ask vaccine manufacturers to expand their clinical trials in younger children.
Experts have thus far said that the benefits of the vaccines far outweigh the rare risk of myocarditis. But citing the heart condition as a worry, regulators in some countries like Britain and Hong Kong have recommended a single dose of the vaccine for adolescents aged 12 to 15 years.
Other studies have also found that vaccination increases the risk of myocarditis. An Israeli study published in August looked at the electronic health records of about 2 million people and found an additional 2.7 cases of myocarditis for every 100,000 vaccinated people, compared with unvaccinated ones.
But the same research found that the risk of myocarditis from having Covid-19 was much higher, resulting in an extra 11 cases of the condition for every 100,000 infected people.
The CDC has estimated that for every million vaccinated boys ages 12 to 17, the shots might cause a maximum of 70 myocarditis cases, but would prevent 5,700 infections, 215 hospitalisations and two deaths.
In the new study, researchers analysed the medical records of 2.4 million members of the Kaiser Permanente Southern California health care system, ages 18 years or older. The participants had received at least one dose of the Pfizer-BioNTech or Moderna vaccines between Dec 14, 2020, and July 20, 2021.
The team identified individuals who were hospitalised within 10 days of receiving a vaccine dose and discharged with a diagnosis of myocarditis. The researchers found 15 cases of confirmed myocarditis in the vaccinated group, 13 of which were observed after the second dose.
None of those affected had a history of heart problems, and none were readmitted to the hospital after being discharged.