Young people who got COVID-19 vaccines have reported higher rates of heart inflammation.
The CDC is investigating whether there's a link between the Pfizer and Moderna shots and these events.
Even if there is a link, doctors said, the risk from COVID-19 is far greater than from the vaccines.
Young people who received the Pfizer-BioNTech or Moderna coronavirus vaccines have reported higher-than-usual rates of heart inflammation and swelling, US health officials said on Thursday.
The findings are preliminary and come from a self-reported database of side effects. The Centers for Disease Control and Prevention hasn't confirmed a link to the vaccines, though researchers are investigating these incidents of heart inflammation, known as myocarditis, and heart swelling, known as pericarditis.
The CDC said it planned to convene an advisory group to discuss the issue next Friday. Earlier this month, Israel's health ministry said there was a "probable link" between Pfizer's vaccine and heart-muscle inflammation in young men.
In interviews with Insider on Thursday, cardiologists and infectious-disease specialists said the risk from the vaccine is still tiny compared with the potential damage from getting infected with the coronavirus. Not all experts are convinced there's a link between the events and the shots.
"It's not a no-brainer that there's an association, because the season for getting myocarditis is around now," Dr. Lorry Rubin, the director of pediatric infectious diseases at Northwell Health's Cohen Children's Medical Center, told Insider.
A group of viruses called enteroviruses is the most common cause of myocarditis, Rubin said. Those viruses typically circulate in the summer.
"That's not the sole cause, but that's the most common infectious trigger for myocarditis, and that's more of a seasonal infection," he said.
All three experts agreed that getting vaccinated is still the best option, even for young and healthy men and boys.
"You definitely should choose vaccination in that age group because it's safer than wild-type-virus infection," said Dr. Leslie Cooper, a cardiologist at the Mayo Clinic who specializes in myocarditis.
Early reporting shows more heart side effects than expected
The preliminary data suggests elevated rates of the heart issues among vaccinated people ages 16 to 24. At a Food and Drug Administration advisory meeting on Thursday, CDC officials shared data covering about 12 million doses given to people in this age range. CDC officials said that they would typically expect to see 10 to 102 cases of these heart events in this group but that 275 cases had been reported.
Most of these cases came after the second dose and were commonly reported in men, according to the preliminary data. The CDC said it was still confirming and investigating these self-reported cases.
The most common symptoms were chest pain and elevated levels of cardiac enzymes. Most of these cases were short-lived; at least 81% of people had already fully recovered from their symptoms, the CDC said.
Even if the cases are linked to the vaccine, experts said, the greater risk, by far, is COVID-19.
"We're talking about an incidence of myocarditis from the vaccine that is 100 times lower than the incidence when you actually get the infection," Dr. Eliot Peyster, a cardiologist at the University of Pennsylvania, told Insider. "So, yes, it's probably twice as likely as people who don't get the vaccine, in a vacuum. But we aren't in a vacuum. We're in a pandemic where young people who get the virus get myocarditis in about one in 300 cases."
Peyster cited a study of college athletes that found that roughly one in 300 had myocarditis after recovering from COVID-19.
The incidence rate among younger people who got the COVID-19 vaccine is still being determined, CDC officials said on Thursday. The agency said it would have more data to present at another meeting next Friday.
A strong immune response to the vaccine could be the culprit
The Pfizer and Moderna shots both use messenger RNA, a new technology that had not been used in a federally approved medicine before the pandemic. Despite its newness, experts said they didn't think these cases stemmed from something unique to the technology.
Peyster said he would guess it was from a robust inflammatory response to the vaccine, particularly after the second dose.
"Any systemic inflammatory condition - any kind - can generate enough inflammatory stuff where you can actually get a little bit of heart-muscle damage just through systemic inflammation," he said.
While more than 140 million Americans have been fully vaccinated, the concerns about younger people come as the vaccines become available to younger populations. Pfizer's vaccine was OK'd last month for children as young as 12 years old. Moderna applied for a similar authorization on Thursday.
Not enough shots have been given and tracked in the 12-to-15-year-old population to know whether these heart side effects could be a concern for that age group. Both vaccine developers have said they're testing their shots in children and infants as young as 6 months.
Experts guess that heart side effects will be less common in younger children
Rubin said he didn't think there was reason for concern among parents deciding whether to get their children vaccinated.
"We don't know if there's a causal relationship, and it appears that the rate is low given the millions of people in the 12-to-25-year-old age group who have gotten this vaccine," he said.
Peyster said he expected that these heart side effects would decrease among younger children. Given the history of myocarditis, he said, it's likely the incidence rate will peak with young adults and adolescents.
The medical experts were unequivocal that the vaccine is clearly the best option overall, for all age groups.
"The evidence says you are much more likely to have heart inflammation from the virus than from the vaccine, even in young, healthy people," Peyster said. "Therefore, the clear recommendation is vaccine is better than no vaccine."
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