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Federal health officials said Thursday they’re seeing rare but higher-than-expected cases of a heart issue called myocarditis among adolescents and young adults who received their second shot of one of the mRNA Covid-19 vaccines.

The data are preliminary and limited, and experts are still trying to determine if there is indeed a link or if there is no connection at all. Officials from the Centers for Disease Control and Prevention and the Food and Drug Administration also said it was too early to establish a rate for how frequently myocarditis — an inflammation of the heart muscle — might occur, particularly for different age groups. It appears the cases were more common in men than women, though the experts noted that men generally have higher rates of myocarditis. It’s also more common in younger adults at a baseline level.


But the presentation to the FDA’s vaccines advisory committee echoed other evidence that the mRNA vaccines from Moderna and Pfizer-BioNTech might raise the risk of myocarditis — and perhaps another condition called pericarditis, which is the inflammation of the tissue around the heart — particularly for young men soon after their second shot. Defense Department officials have also flagged the potential connection, while Israeli health officials have found “a possible link between the second vaccine dose and the onset of myocarditis among young men aged 16 to 30.”

Most people who developed myocarditis, which is generally mild, have already recovered fully, health officials said. Many experts and professional societies stress that the benefits of the vaccines and being protected against Covid-19 still vastly outweigh the risks that accompany the shots. Myocarditis is often caused by an infection.

In their presentations Thursday, health officials highlighted data from different surveillance systems that track possible side effects from vaccines, which each have specific limitations. One overall issue is that 12-to-15-year-olds only recently became eligible for vaccinations, so there is not much age-specific data — particularly on issues that might emerge after the second dose — in the reporting systems.


One data set came from the government’s Vaccine Adverse Event Reporting System, where people can submit reactions after vaccinations.

Overall, among all age groups, there were 573 cases of myocarditis and pericarditis reported after people received their second dose of either mRNA vaccine, compared to 216 after the first dose, according to data presented by Tom Shimabukuro, an immunization safety expert at the CDC.

To determine if there is a connection between a vaccine and a possible side effect, experts compare reported numbers to how many cases of that condition they would expect to see in that population normally.

Among 16-to-17-year-olds, who had received 2.3 million doses, there had been 79 cases of myocarditis or pericarditis reported through VAERS. Based on baseline frequency of the myocarditis and pericarditis, there would have been an expected two to 19 cases in that group.

For 18-to-24-year-olds, who had received 9.8 million doses, there were 196 cases of myocarditis and pericarditis reported, compared to an expected eight to 83 cases.

Shimabukuro noted that not all of the reports submitted to VAERS have been verified, so the true number of myocarditis and pericarditis cases is almost certainly lower. Because of its limitations, experts generally don’t solely look to VAERS to establish causal relationships or risk levels. But Shimabukuro added that more than half of reported cases were among people from 12 to 24 years old, who only accounted for 9% of doses administered.

“We clearly have an imbalance there,” he said.

Additional myocarditis data will be presented at another expert panel meeting scheduled for next Friday, officials said.

In statements, both Moderna and Pfizer noted that a causal connection between the vaccines and myocarditis has not been established. “The company will continue to closely monitor these reports and is actively working with public health and regulatory authorities to further assess this issue,” Moderna said. And in its statement, Pfizer said: “We support CDC’s request for careful assessment of suspected myocarditis and pericarditis cases to further our understanding of the cases reported. With more than 400 million doses of the Pfizer-BioNTech COVID-19 vaccine administered globally, the benefit risk profile of our vaccine remains positive.”

The myocarditis presentation came at a meeting of the FDA’s Vaccines and Related Biological Products Advisory Committee, which was focused on a discussion about what kind of data should be required to support the authorization or approval of Covid-19 vaccines in kids under 12.

As of now, the Pfizer shot’s emergency authorization from the FDA covers people 12 and up. Moderna’s vaccine is authorized for adults 18 and up, though on Thursday the company said it had asked the FDA for an expanded authorization to cover adolescents 12 through 17. Both companies have started clinical trials in children as well, rolling out studies in younger populations with smaller doses being investigated.

The discussion among government health officials and outside experts centered on how to assess the risk-benefit considerations for vaccinating children.

While children — particularly younger children — are generally much less susceptible to the worst effects of Covid-19 than adults, hospitalizations and deaths do occur. There have also been more than 4,000 cases of MIS-C in the United States, an inflammatory disorder that can occur after a case of Covid-19 and can require urgent care.

Vaccinating children could also have benefits beyond protecting them individually. For one, it could help ensure that kids return to school in the fall. It will also build up the level of immunity in the population as a whole, which can stifle transmission and help guard people who remain unvaccinated or who, for health reasons, don’t mount strong immune responses to the shots.

The risk of Covid-19 and how children respond to vaccines can vary greatly by age group; a 2-year-old can look very different from an infection and immune standpoint compared to a 10-year-old compared to a 16-year-old. Such variances require experts to consider granular data as they consider whether to recommend vaccines for particular age groups.

Some of the experts on the panel noted that the risk from Covid-19 to kids — at least for right now — is at a low point, given how far transmission has fallen from other times during the U.S. epidemic. Cody Meissner, a pediatric infectious disease expert at Tufts University School of Medicine, argued that that meant health officials had time to gather more data on the vaccines’ benefits and risks before moving ahead with vaccinating younger groups. Other experts, however, noted that transmission is expected to pick up in the fall and winter, and that the virus was continuing to mutate, so it could make sense to start building protection in children ahead of that.

Overall, the experts and government officials were trying to grapple with the fact that the risk-benefit calculation would change based on community levels of spread, and the fact that societal factors — like getting kids back to school, and in turn helping parents get back to work — were influencing their considerations.

This story has been updated with comments from Moderna and Pfizer.

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