Pfizer, Moderna COVID-19 vaccines will get label warning over rare heart risk, FDA says

COVID-19 vaccine
Labels on COVID-19 vaccines from Pfizer and Moderna will soon include a warning about the risk of heart inflammation, the FDA said Wednesday. (Kunal Mahto/iStock/Getty Images Plus/Getty Images)

Numerous COVID-19 vaccines have faced development delays and safety and supply issues, but the mRNA vaccines from Pfizer-BioNTech and Moderna have enjoyed a relatively unfettered ride to the top of the class.

Now, Wednesday’s news that U.S. experts concluded there is a “likely association” between rare cases of heart inflammation and mRNA vaccines has finally presented the Pfizer-BioNTech and Moderna shots with a dose of adversity. The FDA said it will move quickly to require the companies to add a label warning that young adults and adolescents are susceptible to the side effect.

While the Centers for Disease Control and Prevention (CDC) concluded that the benefits of the vaccine clearly outweigh risks, the verdict could compel those with apprehension to favor adenovirus-type vaccines offered by Johnson & Johnson and AstraZeneca.

Those shots faced safety concerns of their own earlier this year over rare but severe cases of blood clots, which led to fatalities. After an investigation, J&J's vaccine now carries a warning in the U.S.

The latest ruling comes in response to reports of young people, mostly males, developing myocarditis—inflammation of the heart—or pericarditis—inflammation of the heart’s membrane—mostly after a second dose of either mRNA vaccine. The CDC reports that 309 people have been hospitalized, with all but 14 discharged so far. There have been no reported deaths. 

RELATED: Pfizer, AstraZeneca COVID-19 vaccines probed in Europe after reports of heart inflammation, rare nerve disorder 

Heart inflammation has occurred at a rate of 12.6 per million among those aged 12 to 39, according to the CDC’s Vaccine Safety Datalink. More than 138 million Americans have been fully vaccinated with the Pfizer or Moderna shots.

Recipients of the Pfizer vaccine account for more of the heart inflammation cases, but that’s largely a function of the FDA’s approval of the Pfizer shot for those age 12 and up. Moderna's vaccine has yet to be endorsed for those under age 18.

Over the last two months, the Pfizer vaccine has come under scrutiny in Israel and Europe for heart inflammation concerns.

Pfizer on Thursday said the heart inflammation is "an extremely rare side effect, and only an exceedingly small number of people will experience it after vaccination," citing the CDC data.

"Patients have typically rapidly improved with conservative treatment," Pfizer said in a statement. "It is important to note that CDC continues to strongly encourage COVID-19 vaccinations for eligible individuals aged 12 and older. With hundreds of millions of doses of the Pfizer-BioNTech COVID-19 vaccine administered globally, the benefit risk profile of our vaccine remains positive."

Moderna issued a similar statement, adding it "will work closely with FDA and with other regulatory authorities on any appropriate next steps."

RELATED: AstraZeneca COVID-19 vaccine linked to rare blood clots, EMA official says, but benefits still outweigh risks

In a summary for investors, RBC Capital Markets analyst Daniel Busby largely dismissed the news, saying case numbers have received “outsized focus.”

“Nothing that we heard during today’s (CDC) meeting meaningfully changes our outlook for (the Pfizer-BioNTech) vaccine,” Busby wrote. “Specifically, there were no indications that CDC intends to change its current COVID-19 vaccine policy, other than adding greater disclosures/warnings.”

Instead, investors should pay attention when the CDC addresses booster shots, Busby wrote. That will be a “key swing factor” for the vaccine's revenue outlook, he noted.