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An employee shows the Moderna COVID-19 vaccine at Northwell Health's Long Island Jewish Valley Stream hospital in New York on Dec. 21, 2020.Eduardo Munoz/Reuters

Newly released data confirm that the Pfizer-BioNTech and Moderna vaccines are both associated with rare heart problems, and that this side effect is most common after the second shot in adolescent boys and young men. Still, the benefits of vaccination continued to outweigh the risks, scientists said.

The side effects tend to be mild, temporary and uncommon. For every million doses of the second shot given to 12- to 39-year-olds, there were 14 to 20 extra cases of the heart problems, according to the new data, which were presented Monday at a meeting of an independent advisory committee to the U.S. Centers for Disease Control and Prevention.

“The data suggest an association of myocarditis with mRNA vaccination in adolescents and young adults,” Grace Lee, a pediatrician at Stanford and chair of the committee, said at the meeting. “Further data are being compiled to understand potential risk factors, optimal management strategies and long-term outcomes.”

But the benefits of the vaccines are substantial, even for those in the highest risk groups. According to an analysis presented Monday by a CDC scientist, every million doses of the Pfizer vaccine administered to 16- and 17-year-old boys would be expected to cause 73 cases of the heart problems, while preventing more than 56,000 COVID-19 cases and 500 related hospitalizations.

The meeting comes as U.S. federal regulators wrestle with the risk-benefit calculations of vaccinating young Americans. In recent weeks, regulators have grown especially concerned about the risks of two heart conditions: myocarditis, an inflammation of the heart muscle, and pericarditis, an inflammation of the membrane that surrounds the heart.

The new data come from two federal safety-monitoring systems: the Vaccine Adverse Event Reporting System, or VAERS, which collects unverified self-reports, and the Vaccine Safety Datalink, which contains vaccination and health records from nine health care organizations.

As of Aug. 18, VAERS had received more than 2,500 reports of the heart problems in newly vaccinated people. Overall, the risks appear to be elevated for males between 12 and 49 and females between 12 and 29, the data suggest.

The reports were far more common after the second dose of the vaccine than the first, and more common in men than women. For every million doses of the second Pfizer shot, for instance, there were 71.5 reported cases of myocarditis among 16- and 17-year-old boys, compared with just 8.1 for girls of the same age.

The findings aligned with data from the Vaccine Safety Datalink. Using this system, researchers confirmed 56 cases of the heart problems among 12- to 39-year-olds who had received an mRNA vaccine within the previous three weeks. The majority were male.

For every million doses administered to 12- to 39-year-olds, there were an extra 14.4 cases of the heart problems after the second Pfizer shot. There were 19.7 extra cases among 18- to 39-year-olds after the second Moderna shot, which is not yet authorized for children, the scientists calculated.

Most patients were hospitalized for the condition, but recovered quickly; 76 per cent were discharged within two days. There have been no reported deaths.

The scientists also highlighted a study published last week in the New England Journal of Medicine that found that the risk of myocarditis was substantially higher after infection with the virus than after vaccination. “COVID-19 incidence and hospitalization rates are increasing rapidly,” Hannah Rosenblum, a CDC scientist, said at the meeting.

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