A young man in West Virginia receives the vaccine while overlooking the West Virginia Capitol Building at Riggleman Hall.

Stephen Zenner | LightRakete | Getty Images

The number of cases of heart inflammation in 16- to 24-year-olds was higher than expected after receiving their second dose of the Covid-19 vaccine from Pfizer or Moderna, the Centers for Disease Control and Prevention said on Thursday, citing on preliminary data with from his vaccine safety monitoring system.

According to a CDC presentation prepared for a meeting of the Food and Drug Administration Advisory Panel on Thursday, 275 cases of myocarditis or pericarditis, which are inflammatory conditions of the heart, were reported in people aged 16 and over on May 31 registered up to 24 years.

The FDA’s Advisory Committee on Vaccines and Related Biological Products will hold a meeting to discuss the use of Covid-19 vaccines in children 6 months and older.

According to the CDC, the expected number of cases was between 10 and 102.

According to the CDC, a total of 475 cases of myocarditis or pericarditis have been reported in people aged 30 years and younger. Most of the patients hospitalized, or 81% of them, had fully recovered from their symptoms, the agency said. May there are still 15 people in the hospital, three of them in the intensive care unit.

The majority of cases appear to occur in men, and the median time to onset of symptoms is two to three days, according to the CDC.

The CDC’s Vaccine Safety Group announced last month that it is examining heart infections in “relatively few” people who have received Covid vaccinations.

The cases mostly affected adolescents and young adults and usually occurred within four days of the vaccination, the CDC said at the time. The condition has been seen more often in men and most cases appear to be mild, the agency said, although officials are following up on patients.

The CDC is coordinating its investigation with the FDA, which last month approved the Pfizer BioNTech vaccine for adolescents ages 12-15.

“We still don’t know if this is really related to the vaccine,” said Dr. Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, during a virtual question-and-answer event on May 27th. He added that the “handful” of reported cases were “very mild, lasting a day or two,” and usually occurred after a second dose.

Health experts say finding rare side effects once a vaccine or drug is administered to the general population, and if myocarditis is found to be related to the Covid vaccine, the risk is negligible compared to the risks of infection with Covid-19.