The second COVID-19 vaccine dose can nearly triple the chances of myocarditis — a rare heart condition — in young men, though the absolute risk for the condition remains very low, according to study findings published Oct. 4 by JAMA Internal Medicine.
Researchers analyzed data from 2.4 million Kaiser Permanente Southern California members 18 or older who received at least one dose of the Pfizer or Moderna vaccine between Dec. 14 and July 20. Individuals hospitalized within 10 days of receiving a vaccine and discharged with a diagnosis of myocarditis were considered confirmed cases.
Researchers identified 15 cases of confirmed myocarditis, 13 of which were observed after the second dose. Eight patients received the Pfizer vaccine and seven received Moderna.
The study found a higher risk for young men, with about 5.8 myocarditis cases per million second doses in men the average age of 25. The risk was significantly less after the first COVID-19 dose, at 0.8 cases per million, similar to the normal risk for that age group.
None of the individuals with confirmed cases had a history of heart problems, and none were readmitted after discharge.
The study may be an underestimate, since researchers tracked vaccinated individuals for only a short time, and may have also missed those who didn't require hospitalization.
Other studies have also linked COVID-19 vaccines to an increased myocarditis risk. An Israeli study examined EHRs from nearly 2 million people and found an additional 2.7 myocarditis cases for every 100,000 vaccinated individuals, compared to unvaccinated ones. However, the same study also found that the risk of myocarditis from a COVID-19 infection was much higher, with an additional 11 cases for every 100,000 infected people.
COVID-19 patients had 15.7 times the risk for myocarditis compared with patients who didn't have COVID-19, according to CDC data published Aug. 31.