How often do COVID vaccines cause heart problems in kids?

Federal regulators are reviewing data on the link between Moderna's coronavirus vaccine and a rare heart problem in adolescents, the company announced Sunday. That side effect — myocarditis, an inflammation of the heart muscle — has also worried advisers to federal agencies in deliberations regarding use of the Pfizer-BioNTech vaccine in younger children and teenagers.

Scientists advising the Centers for Disease Control and Prevention will review the latest data on the condition at a meeting Tuesday before deciding whether to recommend the Pfizer-BioNTech vaccine for younger children.

So how common is myocarditis, really? And should parents be concerned about vaccinating their children?

Absolutely not, said several experts familiar with the recent studies. While the vaccines made by Pfizer-BioNTech and Moderna do seem to be associated with an increased risk of myocarditis, the absolute risk remains very small. Most cases are mild and resolve quickly.

"If you look at an isolated risk, you could really get yourself very worked up and scared," said Dr. Brian Feingold, an expert on heart inflammation in children at UPMC Children's Hospital of Pittsburgh.

But COVID-19 itself, he noted, is much more apt to damage the heart permanently: "Statistically, that's way more likely."

Myocarditis generally results from infection with a virus or bacteria, and causes symptoms like rapid or irregular heartbeat, chest pain and shortness of breath. Globally, about 10 to 20 people out of every 100,000 develop myocarditis each year, but many others have mild symptoms and may never be diagnosed.

Since the start of the coronavirus pandemic, tens of thousands of children have been hospitalized with COVID, and 657 have died, according to data collected by the CDC.

Some children who are infected with the coronavirus may go on to develop long COVID, remaining ill for months after the initial infection is gone, or multisystem inflammatory syndrome, which has affected at least 5,200 children in the United States.

While the risk of myocarditis after vaccination is real, "those are real numbers that are bigger," Feingold said.

The incidence of myocarditis after vaccination varies with age, sex and dose — and across studies. But the trend so far suggests that the chances are highest after the second dose of an mRNA vaccine in male patients ages 16 to 29.

There are roughly 11 cases of myocarditis for every 100,000 vaccinated male patients in this age group, one study estimated. The odds of myocarditis decrease with age.

Data on adolescents ages 12 to 15 is limited, because the Pfizer-BioNTech vaccine only recently became available to them. But so far, heart problems after vaccination seem to be less common in boys of that age than in older men, said Dr. Paul A. Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia.

This age distribution is similar to that seen among patients with myocarditis caused by viral infections, Offit said.

An FDA scientific advisory committee, on which he sits, met last week to review the Pfizer-BioNTech vaccine's safety in children ages 5 to 11 and ultimately voted to recommend that children be given the vaccine.

"Myocarditis is usually a post-pubertal phenomenon," Offit said, adding, "That made me feel better about the fact that we won't be unpleasantly surprised" by a surge in myocarditis in younger children after they are immunized.

That reassurance may not be enough for some parents. Jeff Gustin, a plant geneticist at the Agriculture Department, said he and two of his sons had elevated heart rates after getting the Pfizer-BioNTech vaccine.

His teenage sons recovered quickly, but Gustin, 42, says he can still feel his heart beating hard, especially when he lies down. Now he is considering a booster of the Johnson & Johnson vaccine, rather than of the Pfizer-BioNTech or Moderna vaccine.

And given the family history, he is hesitant to have his youngest son, who is 11, immunized unless the school district requires it.

But some instances of myocarditis after immunization, like those of Gustin's older sons, may be detected simply because of the intense scrutiny on vaccines, said Dr. James de Lemos, a cardiologist at the University of Texas Southwestern Medical Center in Dallas, who reported one of the first cases in January.

The myocarditis linked to the vaccines is far less frequent and severe compared with that observed in patients with COVID, and it does not seem to cause lasting harm, de Lemos said.

The coronavirus can infect cardiac muscle, as well as the lining of blood vessels, putting the heart and other organs at risk of long-term damage. The virus can also weaken the heart enough to require a transplant, and even cause lethal damage.

By contrast, the myocarditis observed after vaccination is mild and transient. "It's unsettling, but rarely life-threatening," de Lemos said.

Still, some Northern European countries, where the Moderna vaccine was already approved for adolescents, have stopped giving it to people age 30 and younger, at least for the moment. That decision was based on evidence suggesting that the risk of myocarditis is higher with the Moderna vaccine than with the Pfizer-BioNTech vaccine.

When researchers at Kaiser Permanente Northern California directly compared the two vaccines, for example, they found that in men ages 18 to 39 the incidence of myocarditis was elevated after a dose of either vaccine, but more so after Moderna's.

"The rate after Moderna is objectively higher than the rate after Pfizer," said Dr. Nicola P. Klein, director of the Kaiser Permanente Vaccine Study Center. "It's consistent, regardless of which cases we use, or whether it's males only or both sexes."

The risk after both doses of the Moderna vaccine in people ages 18 to 39 was as much as 37 times higher as in the general population, and the rate after two doses of the Pfizer-BioNTech vaccine in those ages 12 to 39 was as much as 19 times higher.

While that may sound alarming, the absolute numbers were still tiny, de Lemos noted. "Thirty times a small number is still a small number," he said. "The math still favors vaccination in adolescents and children."

It's unclear why myocarditis seems to affect boys more often than girls, and whether it is specific to the coronavirus vaccines or a side effect of all mRNA vaccines. The focus on myocarditis after vaccination may also help to understand the condition generally, experts said.

"I'll feel a lot better about this when we have an understanding of the pathogenesis, of exactly what is it that's going on," Offit said. If doctors can identify the cause, he added, "we may be able to treat it to some extent."

This article originally appeared in The New York Times.