Vaccine makers saw stocks fall Friday after President-elect Donald Trump announced that Robert F. Kennedy Jr., a vaccine skeptic, was his choice to lead the Department of Health and Human Services (HHS).

Kennedy Jr. has advocated against processed foods and the use of herbicides. He has long criticized large commercial farms and animal-feeding operations. But his stance on childhood vaccines is perhaps most notable. He has said that he believes in the debunked idea that vaccines cause autism and said in a July podcast interview, “There’s no vaccine that is safe and effective.” However, he has said that he is not anti-vaccine.

On Friday morning, hours after Trump chose Kennedy Jr. as his pick for HHS secretary, Pfizer’s stock dropped 4.3 percent while Moderna’s fell 3.1 percent and Novavax saw a 2.8 percent dip in its stock.