Shares of vaccine makers fell Thursday as President-elect Donald Trump nominated Robert F. Kennedy Jr., a prominent vaccine skeptic, to lead the Department of Health and Human Services. 

The stocks fell in the final hour of trading as reports emerged about Trump’s expected pick. Moderna‘s stock closed more than 5% lower on Thursday, shares of Novavax fell more than 7% and Pfizer‘s stock ended more than 2% lower.

Shares of BioNTech, the German drugmaker that helped develop a Covid vaccine with Pfizer, closed more than 6% lower. British drugmaker GSK, which makes flu shots and several other vaccines, closed roughly 2% lower.

Shares of those companies dipped further in extended trading as Trump confirmed his pick in a post on his platform Truth Social.

Health policy experts have said a second Trump term could allow Kennedy to elevate anti-vaccine rhetoric, which could deter more Americans from receiving Covid shots and routine immunizations that have for decades saved millions of lives and prevented debilitating illnesses.

Pfizer, Moderna and Novavax are still recovering from falling Covid vaccination rates in the U.S., which have dented their profits over the past two years. 

Kennedy’s track record as a vaccine skeptic is extensive. He has long made misleading and false statements about the safety of shots, such as claiming they are linked to autism despite numerous studies going back decades that debunk the association.

Kennedy is the founder of the nonprofit Children’s Health Defense, the most well-funded anti-vaccine organization in the country.

Novavax on Wednesday said the Food and Drug Administration has put a hold on its application for a combination shot targeting Covid and influenza and a standalone flu vaccine, sending the company’s shares down sharply. 

The biotech company’s stock fell nearly 20% on Wednesday. The so-called clinical hold is due to a single report of nerve damage in a patient who received the combination shot in a phase two trial that finished in July last year. 

A clinical hold is an order issued by the FDA to a manufacturer to delay or suspend a proposed clinical investigation on a drug.

It is unclear if the pause will impact Novavax’s ability to start and release data on phase three trials on those vaccines. Still, it appears to be a setback for the biotech company, which is scrambling to bring new products to market as demand for its Covid vaccine plummets worldwide.

Novavax said it was working with the FDA to resolve the clinical hold on its combination shot and standalone flu vaccine. The company said other trials of its Covid and flu shots had not shown any safety concerns related to the type of nerve damage reported in the patient. 

Novavax said it does not believe there’s an established connection that the vaccine had caused the nerve damage in the patient but said it is working to provide more information to the FDA. 

“Our goal is to successfully resolve this matter and to start our Phase 3 trial as soon as possible,” Dr. Robert Walker, Novavax’s chief medical officer, said in a release. 

Public health officials see Novavax’s protein-based Covid vaccine as a valuable alternative for people who don’t want to take mRNA shots from Pfizer and Moderna, which use a newer vaccine method to teach cells how to make proteins that trigger an immune response against Covid.

Novavax’s shot, meanwhile, fends off the virus with protein-based technology, a decades-old method used in routine vaccinations against hepatitis B and shingles.