Number of myocarditis in vaccinated boys slightly higher than thought, Israeli study shows

Boys between the ages of 12 and 15 have a small but increased risk of heart problems after receiving the second dose of the Pfizer-BioNTech vaccine, Israeli researchers reported Wednesday.

Myocarditis, or inflammation of the heart muscle, occurred in 1 of 12,361 boys of this age group within a week of receiving the second dose, the study found.

The cases have been mild and the side effect remains uncommon. In addition, several studies have shown that the risk of myocarditis after Covid is much higher than that after vaccination.

Still, the Israeli figure exceeds the Centers for Disease Control and Prevention’s estimate of 1 case per 16,129 vaccinated adolescents ages 12 to 17. Israeli researchers previously reported that nearly 11 out of every 100,000 men between the ages of 16 and 29 had an increased risk of myocarditis after vaccination.

While the new results are based only on the Pfizer-BioNTech vaccine, other studies have shown that the incidence of myocarditis may be higher with the other mRNA vaccine made by Moderna.

Some European countries have discontinued the use of Moderna vaccine in male adolescents because of the risk. In the United States, only the Pfizer-BioNTech vaccine is approved for use in adolescents and children 5 years of age and older.

Concerns about heart inflammation may have prompted the Food and Drug Administration to ask Pfizer-BioNTech and Moderna to enroll more children in their vaccine trials. Still, Pfizer’s trial of the vaccine included just 567 boys ages 12 to 15 — too few to detect unusual side effects.

The new study tracked hospital admissions for myocarditis between June 2, 2021 and October 20, 2021 through an Israeli health ministry surveillance system. During that time, 404,407 adolescents aged 12 to 15 years received the first dose of vaccine and 326,463 adolescents received the second dose.

The researchers found 18 reports of hospitalization for myocarditis. They ruled out two of the cases because of “reasonable alternative diagnoses,” and two others because they occurred 46 and 70 days after the second dose — too long after the vaccination to be related, the researchers said.

Of the remaining 14, one was in an unvaccinated adolescent, one in an adolescent who received the first vaccine dose within 21 days, and 12 cases within a week of the second dose. The results are published in The New England Journal of Medicine.

All cases were clinically mild and the adolescents were hospitalized for an average of three days. No one was readmitted during 30 days of follow-up.

The researchers estimate that myocarditis occurs in 0.56 male adolescents per 100,000 after the first dose, and 8.09 per 100,000 after the second dose. The corresponding risk in girls of that age was negligible.

The study may have underestimated the risk of myocarditis because the researchers only looked at boys who had been hospitalized for the condition. Myocarditis can manifest as chest pain, shortness of breath, and a feeling of having a fast beating, fluttering or pounding heart, according to the CDC

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