Vaccine stocks continued to fall in premarket trading on Friday, following a sharp decline on Thursday after President-elect Donald Trump announced he would appoint Robert F. Kennedy Jr. to head the Department of Health and Human Services.

Shares of Moderna (MRNA) fell over 8%, while Novavax (NVAX) slipped nearly 2%. Pfizer (PFE) was down almost 5%. Shares of both Moderna and Novavax are now down more than 20% over the past five days.

Trump confirmed the nomination on X after the market close. The stocks continued to fall slightly in after-hours trading.

"The Safety and Health of all Americans is the most important role of any Administration, and HHS will play a big role in helping ensure that everybody will be protected from harmful chemicals, pollutants, pesticides, pharmaceutical products, and food additives that have contributed to the overwhelming Health Crisis in this Country," Trump wrote on X.

Kennedy had been a candidate for president in 2024 before dropping out of the race and pledging his support to Trump.

Trump said during a rally in late October that he would let Kennedy “go wild” on healthcare in the country, and vaccine stocks like Moderna and Novavax have been heading lower since. Thursday's news only furthered that move.

“I’m gonna let him go wild on health. I’m gonna let him go wild on the food. I’m gonna let him go wild on medicines,” Trump said.

Kennedy told NPR in an interview on Nov. 6 that "we're not going to take vaccines away from anybody." But he has been outspoken against vaccines.

Kennedy's nonprofit, Children's Health Defense, has been a vocal critic of childhood vaccinations. And as the AP previously noted, Kennedy himself has said in interviews, "There's no vaccine that is safe and effective."

Josh Schafer is a reporter for Yahoo Finance. Follow him on X @_joshschafer.

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Correction: A previous version of this article misspelled Novavax's name. We regret the error.