Since Americans first began rolling up their sleeves for coronavirus vaccines, health officials have said that those who are immunized are very unlikely to become infected, or to suffer serious illness or death. But preliminary data from seven states hint that the arrival of the Delta variant in July may have altered the calculus. Breakthrough infections are also likely to be most severe among older adults or those who have conditions like obesity or diabetes.

The agency is reviewing several dozen reports that teenagers and young adults may have developed myocarditis after vaccination, officials said. But the agency has not determined whether the vaccine caused the condition.