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Given the reported cases of myocarditis in young people who received mRNA COVID-19 vaccines, FDA will include a warning statement about the risks and characteristics of this rare condition, an agency representative said at CDC's Advisory Committee on Immunization Practices (ACIP) meeting on Wednesday.

Data presented by CDC staff estimated a rate of 12.6 cases per million within 3 weeks of a second dose of either Pfizer's or Moderna's mRNA vaccine for individuals ages 12 to 39. Rates were highest among boys and younger men. The side effect was rare, but when it occurred, it was typically within a week of vaccination.

FDA liaison representative, Doran Fink, MD, PhD, noted the agency will add a warning about the risk of myocarditis or pericarditis following vaccination that states "these events have occurred in some recipients following dose 2, onset of symptoms was several days to a week" and based on limited follow-up, "most cases had a resolution of symptoms."

The warning would also advise anyone experiencing these symptoms to "seek medical attention" and state that information on long-term sequelae with the condition is limited.

CDC staff said they would update their vaccine fact sheets with more comprehensive information in the coming days.

While there was no vote scheduled during the ACIP meeting, the committee seemed to agree with CDC staff that the benefits of COVID-19 vaccination continue to outweigh the risks of vaccination in people ages 12 and older.

HHS released a statement co-signed by CDC Director Rochelle Walensky, MD, and 16 other organizations, including the American Heart Association and American Academy of Pediatrics (AAP), reiterating the safety and efficacy of COVID-19 vaccines.

"The facts are clear: this is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination," the statement said.

ACIP chair, José Romero, MD, agreed, saying he felt the risk/benefit balance was "in favor of benefit" and "I feel very comfortable with that decision."

Romero said the data would be helpful when parents asked for more information about the vaccine.

Matthew Oster, MD, of CDC's COVID-19 Vaccine Task Force, cited prior research on cases of myocarditis and pericarditis, noting that patients were hospitalized for less than a week and treated with anti-inflammatory medication, such as NSAIDs or steroids.

Tom Shimabukuro, MD, of the CDC presented preliminary data from the Vaccine Adverse Event Reporting System (VAERS), specifically among people ages 12 to 29. As of June 11, there were 484 preliminary reports, 323 of which met CDC working case definitions of myocarditis or pericarditis. Of these, 309 people were hospitalized, and almost 80% were known to have recovered from symptoms. Nine people remained hospitalized, two in the ICU.

Interestingly, 14 people were not hospitalized, but were seen in an urgent care, emergency department, or outpatient clinic.

Shimabukuro also noted a higher proportion of observed versus expected cases, specifically among younger males. For example, in boys ages 12-17, there were 128 preliminary cases of myocarditis or pericarditis observed in the week following a second vaccine dose, when up to four might normally be expected in such a time frame. In men ages 18-24, there were 219 reported cases when no more than eight might be expected.

But ACIP member Sarah Long, MD, of Drexel University College of Medicine in Philadelphia, pointed out this post-vaccine myocarditis presented very differently compared to the type of idiopathic myocarditis one expects to see in teenagers.

Whereas post-vaccine myocarditis appears to have a more rapid onset, with more severe symptoms such as chest pain, typical myocarditis presented with more dyspnea and was "a little more subtle in onset." Traditional myocarditis was also much slower to resolve, taking months instead of days.

"Traditional myocarditis is not [multi-inflammatory syndrome in children], and it's not this," she said. "Chances that this ... is related to these vaccines ... is very highly likely."

Booster Dose Discussion Begins

ACIP also began preliminary discussions about the potential data required for COVID-19 vaccine booster doses. CDC staff outlined four data points needed to inform recommendations about booster doses, including risk of COVID-19 complications, exposure, waning immunity, or variants in a population.

Populations that might be targeted for booster doses include residents of long-term care facilities, healthcare personnel, adults ages 65 and older, and immunocompromised people.

However, the ACIP working group said they would only be comfortable recommending boosters when there was evidence of declining vaccine effectiveness, not just antibody response, or evidence of an escape variant that could affect vaccine protection.

Romero, however, thought that might be too conservative, citing influenza as an example where public health officials and clinicians don't wait until there is an outbreak before beginning vaccination against a different type of strain.

"We should begin to consider using these boosters before we have evidence of disease from these variants if [COVID-19] continues to be ... ongoing" in the U.S., he said.

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