The coronavirus vaccines manufactured by Pfizer-BioNTech and Moderna could have caused heart problems in more than 1,200 Americans, including about 500 who were under 30, according to data reported Wednesday by researchers at the Centers for Disease Control and Prevention .

Still, the benefits of vaccination far outweighed the risks, and CDC advisors strongly recommended vaccination for all Americans 12 and older.

The reported heart problems are myocarditis, which is inflammation of the heart muscle; and pericarditis, inflammation of the lining of 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.

Overall, however, the side effect is very rare – only 12.6 cases per million second doses given. The researchers estimated that out of a million second doses given to boys ages 12-17, the vaccines could cause a maximum of 70 cases of myocarditis, 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 of the cases were mild, with symptoms like fatigue, chest pain, and irregular heartbeat that cleared up quickly, the researchers said. Of the 484 cases reported in Americans under the age of 30, the CDC has definitely linked 323 cases to vaccination. The rest are still being investigated.

“These events are really very rare, extremely rare,” said Dr. Brian Feingold, an expert on pediatric heart inflammation at UPMC Children’s Hospital in Pittsburgh. “That has to be seen in the context of illness and morbidity and mortality in connection with Covid.”

Separately, more than a dozen state and professional medical organizations said in a joint statement Wednesday that myocarditis “is an extremely rare side effect and affects an extremely small number of people after vaccination.”

Federal researchers also presented early safety data on Wednesday on the six million vaccine doses given to children ages 12 to 15. The side effects – usually fatigue and pain at the injection site – were similar to those seen in young people aged 16-25.

“So far, the Covid-19 vaccines approved in the USA have shown a high level of safety,” said Dr. Matthew F. Daley, Principal Investigator at Kaiser Permanente Colorado and a member of the Advisory Committee.

The CDC advisors met when the Biden administration publicly admitted that it expects to miss its target of at least partially vaccinating 70 percent of Americans by July 4, will be immunized.

About two in 100,000 people aged 15 to 18 – about two-thirds of them male – are hospitalized with myocarditis each year, according to data presented at the meeting. Patients with the most severe cases may need mechanical assistance, such as a ventilator or a heart transplant.

Even people with mild symptoms may have to abstain from exercise for about six months after recovery. It is unclear what typically causes the condition or why it is more common in young men than women.

Updated

June 23, 2021, 8:51 p.m. ET

The first cases of coronavirus vaccine-related myocarditis were reported in Israel, mostly in young men ages 16-19. Israel recorded 148 cases, 95 percent of them mild, from December to May.

In the United States, too, myocarditis was more common in men and boys: up to 80 percent of the cases diagnosed after the second dose were in men. There was also a marked difference in age, with the side effect becoming more common in people in their late teens and early 20s.

As of June 21, about 318 million doses of coronavirus vaccine had been administered in the United States and 150 million people are considered fully protected. Most symptoms of myocarditis appeared within about four days of the first or second dose.

“We have clear evidence here that vaccinated cases started within the first week,” said Dr. Tom Shimabukuro, a vaccine expert at the CDC, who presented the new data. There is also a dose effect, he said, adding, “The rates are higher with both vaccines after the second dose.”

The vast majority of patients with the side effect made a full recovery, noted Dr. James de Lemos, a cardiologist at the University of Texas Southwestern Medical Center at Dallas, who reported one of the first cases in January.

Covid-19 itself can cause heart problems in young people. A large study of college athletes showed that 2.3 percent of those who recovered from Covid-19 had heart abnormalities associated with myocarditis.

“Even in young men, myocarditis will be far more common if you get Covid than if you get a vaccine,” said Dr. de Lemos.

More than 4,000 children infected with the coronavirus developed a multisystem inflammatory syndrome that includes cardiac symptoms. Some children have also died while none died from the vaccination, noted Dr. Fine gold. “You can say no to the vaccine, but you take different risks.”

The CDC recommends vaccination for all Americans over the age of 12. But on Wednesday officials suggested that anyone who develops myocarditis after the first dose should postpone a second dose until they discuss the risks with a doctor.

The CDC’s recommendations may influence decisions about whether to vaccinate children under 12 years of age when vaccines become available for that age group. Some experts have questioned whether the benefits to children outweigh the potential risks, as the chances of developing serious illness from the virus in young children are small.

Still, the agency reported this month that Covid-19-related hospitalizations among teenagers in the United States were about three times higher than influenza-related hospitalizations in the last three flu seasons.

The total number of infections has fallen sharply since January, but as more adults have been vaccinated, the proportion of children in the total has increased. About a third of the new infections reported in May were in Americans ages 12 to 29, and there have been 316 deaths in that age group since April.

Vaccination becomes an even more pressing priority given more contagious variants of the coronavirus now circulating in the United States, said Dr. Paul Offit, a member of the Food and Drug Administration’s Vaccination Safety Committee, in an interview.

“We’re still a long way from 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 Philadelphia Children’s Hospital. “And you will go into winter when you have a generally underinoculated population.”