Shares of several prominent vaccine makers fell Monday morning after the resignation late Friday evening of the Food and Drug Administrations top vaccine official, Peter Marks.In his resignation letter, Marks cited differences with new Health and Human Services Secretary Robert F. Kennedy Jr., who he said sought subservient confirmation of his misinformation and lies.As head of the FDAs Center for Biologics Evaluation and Research since 2016, Marks has overseen the agencys review of new vaccines, blood products and gene therapies. He helped drive forward Operation Warp Speed under the first Trump administration, which led to the U.S. quickly obtaining safe and effective vaccines for COVID-19.Marks apparent ouster has raised new alarms over how Kennedy, who has for years sowed vaccine misinformation, might change the regulation and review of shots for infectious diseases.Shares in major vaccine makers such as Pfizer and GSK fell around 1% Monday morning, while smaller biotechnology companies like Moderna, BioNTech, Vaxcyte and Novavax declined more steeply. (Vaxcyte also released new trial data Monday that disappointed some analysts on Wall Street.)Kennedys vaccines views were a point of contention during his Senate confirmation hearings. Hes long been known as a vaccine skeptic and, as founder and the former chair of Childrens Health Defense, has helped spread disproven theories about the safety of some available shots.Since taking office, Kennedy has faced criticism for his handling of the measles outbreak that has infected nearly 500 people across the U.S. through last week. (Confirmed totals may undercount the true spread.)Marks addressed the outbreak directly in his letter: The ongoing multistate measles outbreak that is particularly severe in Texas reminds us of what happens when confidence in well-established science underlying public health and well-being is undermined.Two doses of the available vaccine for measles are 97% effective against the disease. While Kennedy has encouraged parents discuss vaccination of their child with their physician, hes also said the decision to vaccinate is a personal one, and touted treatments considered ineffective such as vitamin A.Vaccine advisory meetings have also been disrupted under Kennedy, although one involving Centers for Disease Control and Prevention advisers was rescheduled to next month. It will include discussion of vaccines for COVID, respiratory syncytial virus, human papillomavirus infection and measles.David Risinger, an analyst at Leerink Partners, wrote in a Sunday note to clients that the CDC meeting will be an important catalyst for vaccine developers.Analysts at Jefferies see Marks departure as causing greater uncertainty for vaccine makers, but wrote that the rescheduled meeting agenda signals a commitment to established vaccine evaluation processes.Analysts also viewed President Donald Trumps nomination of Susan Monarez to be CDC director, replacing the withdrawn nomination of vaccine critic David Weldon, as a positive. Monarez has been with the Advanced Research Projects Agency for Health since 2023.The choice of Marks successor may also be another litmus test. Senator Bill Cassidy, R.-La., who provided the key vote for confirming Kennedy, said on X Saturday that Marks should be replaced with someone of similar stature and credibility amongst the scientific community, who will lead without bias.Martin Makary, confirmed as FDA Commissioner last week, would typically control who is named to fill Marks position.Moderna, BioNTech, Novavax and HHS did not respond to requests for comment from BioPharma Dive. '