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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. On Wednesday, Sweden and Denmark joined the ranks, announcing that adolescents should get only one jab of the Moderna vaccine.
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 next week 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.
It is too early to know whether myocarditis might permanently weaken the hearts of some people after vaccination, said Dr. Jeremy Brown, an expert in respiratory diseases at University College London and a member of Britain’s vaccine advisory group.
“That makes it very hard for us to make the absolute statement that it’s totally safe to give this vaccine,” Dr. Brown said. “We need some feel for what the long-term consequences of the myocarditis might be.”
The urgency of fully vaccinating children with two doses has to be weighed for each country’s particular situation, experts said. In Britain, high rates of vaccination among older and high-risk adults have helped to keep hospitals mostly free of patients severely ill with Covid-19.
“The chance of getting severe Covid in a healthy 12- to 15-year-old is almost negligible,” Dr. Brown said. “Against that, you have to make sure that the vaccine that you’re giving is utterly safe.”
Some experts have argued that immunizing children would help to sever chains of transmission and contain the virus. But immunizing children to protect others — when there may be a risk to the recipient, however small — was indefensible, Dr. Brown said.
“You don’t vaccinate a 15-year-old to prevent them infecting other adults — that’s not morally, ethically the right thing to do,” he said.
In Hong Kong, the argument for double-dosing adolescents is even weaker than in Britain, said Benjamin Cowling, an epidemiologist at the University of Hong Kong.
Hong Kong has recorded only 213 deaths and just over 12,000 cases of Covid-19 since the start of the pandemic, with fewer than 10 cases per day since April. So the risk of myocarditis, however rare, outweighs the benefit of fully vaccinating adolescents, Dr. Cowling said.
Clinical trials of the vaccine in children are not large enough to detect rare side effects like myocarditis, he added. “You’d only see it when it goes to the population level, and then it’s too late.” Whether to offer second doses to children “does need a careful consideration.”
But the United States is not in same position as other countries, noted Dr. Jeffrey Duchin, an infectious diseases physician and a nonvoting member of the C.D.C.’s advisory group on vaccines.
About 2,000 Americans are dying every day, and hospitals in many parts of the country are still packed. “We’ve had a significant impact on our pediatric population,” Dr. Duchin said.
Nearly 900,000 children have been hospitalized with Covid-19 since the pandemic began, and about 520 have died. Some children have developed so-called long Covid-19, in which symptoms can persist for months, and more than 4,000 have been diagnosed with a dangerous condition called multisystem inflammatory syndrome in children.
“All the data that we have so far suggests the disease itself is significantly worse than the vaccine side effects,” Dr. Duchin said. Given all of that, a small risk of myocarditis is well worth taking, he said, and two doses are justified.
Dr. Duchin said he also had some concerns that one dose of the vaccine might not shield children against infection or illness — at least, not for long. “I just have not seen data that would suggest that one dose would have a durable and high level of protection,” he said.
All of these concerns, as well as the data on myocarditis, should inform a national conversation about the wisdom of offering one dose versus two shots to adolescents, some experts said.
“There hasn’t been enough discussion about the potential harms of vaccination, because everybody is very, very sensitive about hesitancy and doesn’t want to give any fuel to anti-vaccination campaigns,” Dr. Cowling said.
In the United States, in particular, many public health experts have been reluctant to voice concerns about the vaccines, Dr. Gellad said: “No one wants to introduce any doubt that kids should be vaccinated.”
“But I think there are ways to talk about it that will appeal to people who are hesitant.”
Kristina Rogers, a 51-year-old mother of two in Oklahoma, said she would welcome the option to give her 12-year-old daughter only one dose of the vaccine.
Ms. Rogers, who is fully immunized, worries that not enough was known about the vaccines’ long-term effects in children and said she wished there were more open discussion.
Ms. Rogers has diabetes and developed chronic kidney disease after a severe bout with Covid last year. She lost her brother-in-law to Covid a year ago.
But the two doses of vaccine left her feeling flattened and fatigued, as well, and she worried the shots might prove too much for her children. Her children wear masks to school and wash their hands regularly, but she and her husband are not yet ready to vaccinate them.
“The last thing you want to do is mess with their ticker, man — that’s what makes them go,” she said. “I would be more up for the one dose, if that was an option.”