In a 2015 study of N.C.A.A. players, researchers showed that risks varied by sport and gender. Male Division I basketball players faced up to 10 times the risk of sudden cardiac death compared with all N.C.A.A. athletes. Male athletes faced a higher risk than women, and Black men faced a higher risk than men overall, the study found.

“Folks who maintain good amounts of exercise throughout their life span, they end up at lower risk of having these sudden events,” said Dr. Meagan Wasfy, a sports cardiologist at Massachusetts General Hospital, who published a review of the study. “But for that small period of time where you’re exercising, that risk goes up.”

One list circulating about the claim included 543 unconfirmed reports of athletes around the world who have died or faced “serious issues” since 2021. It was published by the anti-vaccine website Good Sciencing.

The list was based on a mix of news reports and entries on the Vaccine Adverse Event Reporting System, or VAERS, which relies on self-reported cases from patients and doctors. Most news reports did not mention whether the deceased had been vaccinated. Health officials warn against using VAERS to make determinations about vaccine risks.

There is a known and uncommon vaccine side effect, called myocarditis, that involves an inflammation of the heart muscle. Men and boys who receive the Covid-19 vaccine are at higher risk of developing the condition, which can lead to chest pain and shortness of breath. In very rare cases, it can lead to more severe complications, including death.

Doctors say the risk of developing myocarditis after getting vaccinated appears low and most people afflicted with the condition quickly recover. One study found that boys and young men infected with Covid-19 are up to six times more likely to develop myocarditis than people who received the vaccine.

As of Jan. 20, VAERS had received 2,132 preliminary reports of myocarditis or pericarditis (an inflammation of the outer lining of the heart) among vaccinated people 30 or younger, according to the Centers for Disease Control and Prevention. More than 48 million people ages 5 to 24 have received at least one dose of the vaccine.

Health care providers are required to report any death after vaccination, even if there is no sign it was caused by the vaccine. VAERS has received 11,657 reports of someone dying at some point after receiving the vaccine, representing 0.002 percent of all vaccinated people.