At the same time, regulators have been concerned by data from Nordic countries and elsewhere suggesting that young male Moderna recipients may face heightened risks of myocarditis, or inflammation of the heart muscle — a side effect that has also been tied to the Pfizer vaccine. Scientists have said the absolute risk still remains very small, most cases are mild and resolve quickly and that Covid-19 can also trigger myocarditis.
Concerns about myocarditis are behind the F.D.A.’s decision to wait to authorize the vaccine for adolescents.
The F.D.A. said on Friday that in reviewing broader booster eligibility for Moderna’s and Pfizer’s vaccines, it had examined recent virus cases and real-world data on the risks of myocarditis and concluded that the benefits of a booster shot outweighed the risks. The F.D.A. also noted that both companies were conducting additional studies on myocarditis risk.
At the heart of the booster debate is the question of what vaccines are supposed to do. Critics of the administration’s policy argue that despite some degree of waning protection, the vaccines are still fulfilling their mission of protecting against severe disease and hospitalization.
Booster advocates like Dr. Fauci counter that the vaccines should also protect against symptomatic disease, especially since some patients avoid hospitalization but suffer long-term consequences.
“I don’t know of any other vaccine that we only worry about keeping people out of the hospitals,” Dr. Fauci said at a White House briefing Wednesday. “I think an important thing is to prevent people from getting symptomatic disease,” including younger people.
In recent weeks, state after state has moved to allow booster shots for all adults, including Massachusetts, Rhode Island, Kansas, Kentucky, Maine, Vermont, Arkansas, California, Colorado and New Mexico.
A host of other countries have taken the same approach, including Australia, Brazil, Canada, Germany, Hungary, Israel, Japan, Norway and Saudi Arabia. Regulators for the European Union have authorized booster doses of both Moderna and Pfizer-BioNTech for all adults.
Kitty Bennett contributed research.