To some experts, the clustering of cases soon after the second shot suggests that they are a result of the immune response, which, after being primed by the first dose, goes too far and causes the inflammation. One hypothesis for people who develop myocarditis or pericarditis after the first shot is that some had an initial Covid-19 infection, so the first shot was the equivalent of a second exposure, akin to the second dose in people who hadn’t had Covid-19.

David Juurlink, the head of the division of clinical pharmacology at the University of Toronto, has concerns that if it’s the immune response that’s triggering the inflammation, then it might occur at higher rates after the third dose. Beyond that, he also thinks it’s possible that cases could be more severe.

“I do worry that for young men who’ve had two doses and get a third, the risk might be higher,” Juurlink said.

In granting full approval to the Pfizer shot last month, the Food and Drug Administration said the rates of myocarditis and pericarditis in vaccinated boys 16 to 17 years old could be as high as 1 in every 5,000, but that the benefits of the shots still outweighed the risks.