More than 1,200 cases of heart inflammation in adolescents and young adults following Pfizer's or Moderna's COVID vaccine have been reported to health authorities in the United States, the Centers for Disease Control and Prevention (CDC) said on June 23.
The bulk of the reports submitted to the Vaccine Adverse Event Reporting System, a passive reporting system run by two top health agencies, dealt with myocarditis or pericarditis experienced after a second dose of a messenger RNA-based COVID-19 vaccine.
Most of the cases were seen after the Pfizer-BioNTech shot. The others were seen after a Moderna shot.
Continuing a trend first seen earlier this year, the majority of the patients were male. About 40 percent were 29 years old or younger.
Of the 323 cases meeting the CDC's case definition, 309 were hospitalized. Nine are still hospitalized.
The case rate, based on reports to the reporting system, is higher than expected in young males.
For males between the ages of 12 to 17, the expected number of cases following dose one using a 21-day window was two to 21. The observed incidence was 32 through June 11.
For males between the age of 18 and 24, the expected number of cases using the same parameters was three to 34. The observed incidence was 47.
Following dose two using a 21-day window, the observed rate was much higher in males up to 39 years old. It was also higher for females from 12 to 24 years old.
"It does appear that the mRNA vaccines may be a trigger for myocarditis," Dr. Matthew Oster, a pediatric cardiologist at Children's Healthcare of Atlanta, told the panel in a presentation that took place before Shimabukuro was set to speak.
Additionally, Dr. Grace Lee, co-chair of the COVID-19 Vaccine Safety Technical Work Group, said later that data available to date "suggest likely association of myocarditis with mRNA vaccination in adolescents and young adults."
Pfizer and Moderna didn't immediately return requests for comment.
Experts say the panel could end up recommending that certain age populations shouldn't get mRNA-based vaccines or that youths only get one shot, not two.