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The US Food and Drug Administration (FDA) is adding a warning to mRNA COVID-19 vaccines’ fact sheets from Pfizer and Moderna as medical experts continue to investigate cases of heart inflammation, which are rare but are more likely to occur in young men and teen boys.

Doran Fink, MD, PhD, deputy director of the FDA’s Division of Vaccines and Related Products Applications, told a Centers for Disease Control and Prevention (CDC) expert panel Wednesday that the FDA is finalizing language on a warning statement for healthcare providers, vaccine recipients, and parents or caregivers of teens.

The incidents are more likely to follow the second dose of the Pfizer or Moderna vaccine, with chest pain and other symptoms occurring within several days to a week, the warning will note.

“Based on limited follow-up, most cases appear to have been associated with resolution of symptoms, but limited information is available about potential long-term sequelae,” Fink said, describing the statement to the Advisory Committee on Immunization Practices (ACIP), independent experts who advise the CDC.

“Symptoms suggestive of myocarditis or pericarditis should result in vaccine recipients seeking medical attention,” he said.

Benefits Outweigh Risks

Although no formal vote occurred after the meeting, the ACIP members delivered a strong endorsement for continuing to vaccinate 12- to 29-year-olds with the Pfizer and Moderna vaccines despite the warning.

“To me it’s clear, based on current information, that the benefits of vaccine clearly outweigh the risks,” said ACIP member Veronica McNally, president and CEO of the Franny Strong Foundation in Bloomfield, Michigan, a sentiment echoed by other members.

As ACIP was meeting, leaders of the nation’s major physician, nurse, and public health associations issued a statement supporting continued vaccination: “The facts are clear: this is an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination.

“Importantly, for the young people who do, most cases are mild, and individuals recover often on their own or with minimal treatment. In addition, we know that myocarditis and pericarditis are much more common if you get COVID-19, and the risks to the heart from COVID-19 infection can be more severe.”

ACIP heard the evidence behind that claim. According to the Vaccine Safety Datalink, which contains data from more than 12 million medical records, myocarditis or pericarditis occurs in 12- to 39-year-olds at a rate of 8 in a million after the second Pfizer dose and 19.8 per million after the second Moderna dose.

The CDC continues to investigate the link between the mRNA vaccines and heart inflammation, including any differences between the vaccines.

Most of the symptoms resolved quickly, said Tom Shimabukuro, deputy director of CDC’s Immunization Safety Office. Of 323 cases analyzed by the CDC, 309 were hospitalized, 295 were discharged, and 218, or 79%, had recovered from symptoms.

“Most post vaccine myocarditis has been responding to minimal treatment,” pediatric cardiologist Matthew Oster, MD, MPH, from Children’s Healthcare of Atlanta, Georgia, told the panel.

COVID “Risks Are Higher”

Overall, the CDC has reported 2767 COVID-19 deaths among people aged 12 to 29 years, and there have been 4018 reported cases of the COVID-linked inflammatory disorder MIS-C since the beginning of the pandemic.

That amounts to one MIS-C case in every 3200 COVID infections — 36% of them among teens aged 12 to 20 years and 62% among children who are Hispanic or Black and non-Hispanic, according to a CDC presentation.

The CDC estimated that every one million second-dose COVID vaccines administered to 12- to 17-year-old boys could prevent 5700 cases of COVID-19, 215 hospitalizations, 71 intensive care unit admissions, and two deaths. There could also be 56 to 69 myocarditis cases.

The emergence of new variants in the United States and the skewed pattern of vaccination around the country also may increase the risk to unvaccinated young people, noted Grace Lee, MD, MPH, chair of the ACIP’s COVID-19 Vaccine Safety Technical Subgroup and a pediatric infectious disease physician at Stanford Children’s Health.

“If you’re in an area with low vaccination, the risks are higher,” she said. “The benefits [of the vaccine] are going to be far, far greater than any risk.”

Individuals, parents, and their clinicians should consider the full scope of risk when making decisions about vaccination, she said.

As the pandemic evolves, medical experts have to balance the known risks and benefits while they gather more information, said William Schaffner, MD, an infectious disease physician at Vanderbilt University School of Medicine in Nashville, Tennessee, and medical director of the National Foundation for Infectious Diseases.

“The story is not over,” Schaffner told Medscape Medical News. “Clearly, we are still working in the face of a pandemic, so there’s urgency to continue vaccinating. But they would like to know more about the long-term consequences of the myocarditis.”

Booster Possibilities

Meanwhile, ACIP began conversations on the parameters for a possible vaccine booster. For now, there are simply questions: Would a third vaccine help the immunocompromised gain protection? Should people get a different type of vaccine — mRNA versus adenovirus vector — for their booster? Most important, how long do antibodies last?

“Prior to going around giving everyone boosters, we really need to improve the overall vaccination coverage,” said Helen Keipp Talbot, MD, associate professor of medicine at Vanderbilt University School of Medicine. “That will protect everyone.”

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