Federal officials are reviewing nearly 800 cases of rare heart problems following immunization with the coronavirus vaccines made by Pfizer-BioNTech and Moderna, according to data presented at a vaccine safety meeting on Thursday.
Not all of the cases are likely to be verified or related to vaccines, and experts believe the benefits of immunization far outweigh the risk of these rare complications. But the reports have worried some researchers. More than half of the heart problems were reported in people ages 12 to 24, while the same age group accounted for only 9 percent of the millions of doses administered.
“We clearly have an imbalance there,” said Dr. Tom Shimabukuro, a vaccine expert at the Centers for Disease Control and Prevention who presented the data. Advisers to the agency will meet on June 18 to explore the potential links to the complications: myocarditis, inflammation of the heart muscle, and pericarditis, inflammation of the membrane surrounding the heart.
About two-thirds of the cases were in young males, with a median age of 30 years. The numbers are higher than would be expected for that age group, officials said, but have not yet been definitively linked to the vaccines.
As of May 31, 216 people had experienced myocarditis or pericarditis after one dose of either vaccine, and 573 after the second dose. Most cases have been mild, but 15 patients remain in hospitals. The second dose of the Pfizer-BioNTech vaccine was linked to about twice as many cases as the second dose of the vaccine made by Moderna.
There were 79 reported cases of the heart problems among those 16 or 17 years old, compared with a maximum of 19 cases expected for that group. And in the group of young people ages 18 to 24, there were 196 cases, compared with an expected maximum of 83.
But the true incidence may be lower, Dr. Shimabukuro said. Immunizations of younger teenagers began only last month, and data from that age group in particular are limited.