MW Vaccine stocks take a hit as FDA official points to link between COVID-19 vaccines and rare heart condition in young men

By Jaimy Lee

Vinay Prasad, a top regulator at the FDA, did not detail the data in the internal memo that led to the conclusion

Pfizer and BioNTech developed the COVID-19 vaccine Comirnaty.

Shares of Moderna and BioNTech were down on Monday morning, likely in response to a FDA official's memo outlining a link between COVID-19 mRNA vaccines and myocarditis deaths in 10 children.

Concern about that link, particularly relating to teenage boys and young men, has been a key argument against administering mRNA-based COVID vaccines in children.

Vinay Prasad, the head of the Food and Drug Administration's Center for Biologics Evaluation and Research, reportedly issued a memo to staff last week that said the regulator plans to acknowledge that 10 children died after getting the vaccine. (The memo was widely reported by media outlets and shared publicly by Adam Feuerstein, a biotechnology journalist.)

The FDA has not publicly shared the internal research cited in the memo, although Prasad said it comes from the Vaccine Adverse Event Reporting System, which gathers reports of adverse events from the public, healthcare professionals and manufacturers.

During the pandemic, Prasad gained an online following for his opposition to vaccine mandates and concern about myocarditis risk from the COVID-19 vaccines.

Other studies have found that the risk of myocarditis from a COVID-19 infection is higher than from vaccination.

Research published in medical journals indicates that myocarditis after a COVID-19 mRNA vaccine is rare, although it is more likely to occur in boys within the first week of receiving the second dose. The condition often resolves on its own, according to a letter published in JAMA Pediatrics in 2023 by Paul Offit, a leading vaccine researcher and director of the Vaccine Education Center at Children's Hospital of Philadelphia.

Pfizer $(PFE)$ in September shared global data that also show myocarditis is rare but is most common in young men within two weeks of a second dose of the primary series of mRNA COVID-19 shots. The drugmaker said data show that the condition is less common after a booster than after the first two shots.

Pfizer and BioNTech developed the COVID-19 vaccine Comirnaty.

Moderna shares (MRNA) were down about 6% Monday, while BioNTech's stock $(BNTX)$ fell 4%. Both companies developed and now market mRNA vaccines for COVID-19. Dynavax $(DVAX)$, which just made a deal with Vaxart (VXRT) for its experimental COVID-19 vaccine, saw its shares drop 5%. The stock of another vaccine biotech called Vaxcyte $(PCVX)$ was also down 6%.

This is not the first time Prasad has made changes affecting the mRNA COVID-19 vaccines. In June, the FDA added the risks of myocarditis and pericarditis to Comirnaty and SpikeVax, Moderna's shot.

"Our interpretation of the memo is that CBER will focus its efforts on the younger 12- to 24-year-old male population for newly approved COVID-19 vaccines where the myocarditis risk is highest," William Blair analysts told investors.

Wall Street is now considering how Prasad's memo, which also reportedly mentions vaccines for flu and pneumonia, may affect drugmakers that aren't just working on mRNA technology and COVID-19 vaccines. The FDA official also called for different endpoints in clinical trials for vaccines.

As of Nov. 21, 5.8% of children and 13.8% of adults in the U.S. have received a COVID-19 vaccine during this respiratory-disease season, according to the Centers for Disease Control and Prevention.

-Jaimy Lee

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December 01, 2025 12:35 ET (17:35 GMT)

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