Myocarditis, a rare side effect, occurs mostly after the second dose. So in some countries, officials are trying out single doses for children.
Even as parents in the United States wrestle with difficult questions over vaccinating their children against the coronavirus, families in other countries have been offered a novel option: giving children just one dose of the vaccine.
Officials in Hong Kong as well as in Britain, Norway and other countries have recommended a single dose of the Pfizer-BioNTech vaccine for children ages 12 and older — providing partial protection from the virus, but without the potential harms occasionally observed after two doses.
Health officials in those countries are particularly worried about increasing data suggesting that myocarditis, an inflammation of the heart, may be more common among adolescents and young adults after vaccination than had been thought.
The risk remains very small, and significant only after the second dose of an mRNA vaccine. But the numbers have changed the risk-benefit calculus in countries where new infections are mostly lower than in the United States.
Advisers to the Centers for Disease Control and Prevention reviewed data on myocarditis in June, and unanimously voted to recommend the vaccine for children ages 12 and older, saying the benefits far outweighed the risk.
Agency research has estimated that for every million vaccinated boys ages 12 to 17 in the United States, the shots might cause a maximum of 70 myocarditis cases, but they would prevent 5,700 infections, 215 hospitalizations and two deaths. Studies have also shown that the risk of heart problems after Covid-19 is much higher than after vaccination.
Myocarditis was among the concerns that led the Food and Drug Administration to ask vaccine makers this summer to increase the number of children in clinical trials. The issue is likely to be the focus of intense discussion when agency advisers meet later this month to review the evidence for vaccinations of children ages 5 to 11.
The latest analysis, which was published on Wednesday in The New England Journal of Medicine, found that the incidence of myocarditis after vaccination in Israel was highest among males aged 16 to 29. About 11 of every 100,000 males in that age group developed the condition a few days after being vaccinated, a rate higher than most earlier estimates. (The risk was negligible in females of any age.)
Of the 54 cases identified in the study, one was severe enough to require ventilation. Another patient with a history of heart disease died of an unknown cause soon after discharge from the hospital.
Of the 14 patients in the new study who showed heart abnormalities when they were admitted to the hospital, 10 still had some signs of problems when they were discharged. But when the patients were examined again a few weeks later, all five of those for whom results were available appeared to have fully recovered.
A second study, which was also published in the journal, found that boys between 16 and 19 years of age had the highest incidence of myocarditis after the second dose — nine times as high when compared with unvaccinated boys of the same age during the same time.
Health officials in other countries plan to revisit the one-dose strategy as more safety information becomes available, and they may choose to proceed with second shots. But the possibility of deferring the second jab has not received enough attention in the United States, said Dr. Walid Gellad, a drug safety expert at the University of Pittsburgh.
“In the U.S., people have not wanted to talk about it, for unclear reasons,” Dr. Gellad said. “Parents who are hesitant may appreciate the fact that the risk of side effects is actually much lower for one dose than it is for two doses.”
Serious side effects have primarily been seen in boys, so the dosing calculus should be different for boys and girls, he added.