Heart Problems After Vaccination Are Very Rare, Federal Rese
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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 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 and pericarditis. 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. 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.

Separately, more than a dozen federal and professional medical organizations said in a joint statement that myocarditis “is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination.” Federal researchers also presented early safety data regarding the six million doses of vaccines administered to children ages 12 to 15. The side effects were similar to those observed in young people ages 16 to 25.

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, myocarditis has been more common in men and boys: Up to 80 percent of cases diagnosed after the second dose were in males.

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. 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.

The C.D.C. recommends vaccination for all Americans over age 12. But on June 23, 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.