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.

Shares fell in the final hour of trading as reports emerged of Trump’s expected choice. Modern‘s stock closed more than 5% lower on Thursday, the shares of Novavax fell by more than 7% and Pfizer‘s stock ended more than 2% lower.

Shares of BioNTechthe German drug manufacturer that helped develop a Covid vaccine together with Pfizer, closed more than 6% lower. British drug manufacturer GSKwhich makes flu shots and several other vaccines, closed about 2% lower.

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

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

Pfizer, Moderna and Novavax are still recovering from declining Covid-19 vaccination rates in the US, which have hit their profits over the past two years.

Kennedy’s record as a vaccine skeptic is extensive. He has long made misleading and false statements about the safety of injections, such as claiming they are linked to autism, despite numerous studies dating back decades debunking this link.

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