The coronavirus vaccines made by Pfizer-BioNTech and Moderna may have caused heart problems in more than 1,200 Americans, including about 500 who were younger than age 30, according to data reported on Wednesday by researchers at the Centers for Disease Control and Prevention.
Still, the benefits of immunization greatly outweighed the risks, and advisers to the C.D.C. strongly recommended vaccination for all Americans 12 and older.
The heart problems reported are myocarditis, an inflammation of the heart muscle; and pericarditis, inflammation of the lining around the heart. The risk is higher after the second dose of an mRNA vaccine than after the first, the researchers reported, and much higher in men than in women.
But overall, the side effect is very uncommon — just 12.6 cases per million second doses administered. The researchers estimated that out of a million second doses given to boys ages 12 to 17, the vaccines might cause a maximum of 70 myocarditis cases, but would prevent 5,700 infections, 2,215 hospitalizations and two deaths.
Agency researchers presented the data to members of the Advisory Committee on Immunization Practices, which makes recommendations on vaccine use in the United States. (The scientists grouped pericarditis with myocarditis for reporting purposes.)
Most cases were mild, with symptoms like fatigue, chest pain and disturbances in heart rhythm that quickly cleared up, the researchers said. Of the 484 cases reported in Americans under age 30, the C.D.C. has definitively linked 323 cases to vaccination. The rest remain under investigation.
“These events are really very rare, extremely rare,” said Dr. Brian Feingold, an expert on heart inflammation in children at the UPMC Children’s Hospital of Pittsburgh. “That needs to be taken in context with illness and morbidity and mortality related to Covid.”
Separately, more than a dozen federal and professional medical organizations said in a joint statement on Wednesday that myocarditis “is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination.”
Federal researchers on Wednesday also presented early safety data regarding the six million doses of vaccines administered to children ages 12 to 15. The side effects — usually fatigue and pain at the injection site — were similar to those observed in young people ages 16 to 25.
“To date, the Covid-19 vaccines authorized in the U.S. have demonstrated a high degree of safety,” said Dr. Matthew F. Daley, a senior investigator at Kaiser Permanente Colorado and a member of the advisory committee.
The C.D.C. advisers met as the Biden administration publicly acknowledged that it expected to fall short of its goal of getting 70 percent of Americans at least partly vaccinated by July 4. The shortfall, officials said on Tuesday, resulted in part from reluctance among younger Americans to be immunized.
About two out of every 100,000 people aged 15 to 18 — about two-thirds of them male — are hospitalized each year with myocarditis, according to data presented at the meeting. Patients with the most severe cases may require mechanical support, like a ventilator, or a heart transplant.
Even those with mild symptoms must refrain from exercise for about six months after recovery. It’s unclear what typically causes the condition, or why it’s more common in young men than in women.
The first cases of myocarditis linked to coronavirus vaccines were reported in Israel, mostly among young men aged 16 to 19. Israel recorded 148 cases from December to May, 95 percent of them mild.
In the United States, too, myocarditis has been more common in men and boys: Up to 80 percent of cases diagnosed after the second dose were in males. There has also been a clear age difference, with the side effect clustered in individuals in their late teens and early 20s.
About 318 million coronavirus vaccine doses have been administered in the United States as of June 21, and 150 million people are considered to be fully protected. Most of the myocarditis symptoms emerged within about four days of the first or second dose.
“We have clear evidence here of onset for the vaccinated cases within the first week,” said Dr. Tom Shimabukuro, a vaccine expert at the C.D.C. who presented the new data. There is also a dose effect, he said, adding, “The rates are higher for both vaccines after dose two.”
The vast majority of patients with the side effect fully recover, noted 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.
Covid-19 itself may cause heart problems in young people. A large study of collegiate athletes showed that 2.3 percent of those who had recovered from Covid-19 had heart abnormalities consistent with myocarditis.
“It’s going to be manifold more common to get heart muscle inflammation from getting Covid than you would from getting a vaccine, even in young men,” Dr. de Lemos said.
More than 4,000 children infected with the coronavirus developed multisystem inflammatory syndrome, which includes heart symptoms. Some children have also died, while none have died from vaccination, Dr. Feingold noted. “You can say no to the vaccine, but you’re assuming other risks.”
The C.D.C. recommends vaccination for all Americans over age 12. But on Wednesday, officials suggested that anyone who develops myocarditis after the first dose should defer a second dose until they discuss the risks with a health care provider.
The C.D.C.’s recommendations may influence decisions about whether to immunize children younger than 12 as vaccines become available for that age group. Some experts have questioned whether the benefits to children outweigh the potential risks, given the low odds of developing serious illness from the virus in young children.
Still, the agency reported this month that the number of Covid-19-related hospitalizations among adolescents in the United States was about three times higher than hospitalizations linked to influenza over three recent flu seasons.
The overall number of infections has steeply dropped since January, but as more adults have been vaccinated, the proportion of children in the total has risen. About one-third of new infections reported in May were in Americans aged 12 to 29, and 316 deaths have been recorded in this age group since April.
Vaccination is becoming an even more urgent priority, given more contagious variants of the coronavirus now circulating in the United States, Dr. Paul Offit, a member of the Food and Drug Administration’s vaccine safety committee, said in an interview.
“We are not close to being near where we need to be” in terms of the percentage of Americans who should be vaccinated, said Dr. Offit, who is also a pediatrician at the Children’s Hospital of Philadelphia. “And you’re going to head into winter when you’re going to have a generally undervaccinated population.”