The weekend’s resignation announcement of FDA official Peter Marks hit biotech stocks on Monday and sent Wall Street into a frenzy of concern over the future of biopharma approvals in the near term.

Marks was the director of the Center for Biologics Evaluation and Research, referred to as CBER, within the FDA. Under his leadership, the FDA approved COVID-19 vaccines during the pandemic as well as the latest RSV (respiratory syncytial virus) vaccines.

Vaccine stocks such as Moderna (MRNA) fell more than 10% in early trading Monday.

In his resignation letter, Marks raised concerns about the anti-vaccine beliefs of Health and Human Services (HHS) Secretary Robert F. Kennedy Jr. and whether the new FDA head, Marty Makary, would be able to stand up to them.

“Efforts currently being advanced by some on the adverse health effects of vaccination are concerning,” Marks wrote. “Undermining confidence in well-established vaccines … is irresponsible, detrimental to public health, and a clear danger to our nation’s health, safety, and security.”

Several other vaccine biotech stocks slid in on Monday. Novavax (NVAX) was down more than 8% while BioNTech (BNTX) was down more than 7%. Dynavex (DVAX) declined more than 2%, and Vaxcyte (PCVX) fell the most, dropping more than 54% early Monday.

Read more about Moderna stock and today’s market action here.

Large-cap vaccine makers were trading flat or only slightly down in premarket trading. Some companies with near-term decisions on novel therapies were also down. Gilead (GILD) slid more than 4% in premarket trading but pared losses to 1.5% after the open.

Wall Street expressed concerns about the impact Marks’s resignation would have, though many experts and analysts said the move was not a surprise.

“This was always a risk given his leadership in Operation Warp Speed,” Jefferies analyst Michael Yee wrote in a note to clients. “We now see heightened risk to rare disease [companies], … note many have already traded down >20% YTD given heightened fears about his leaving and the future of FDA regulatory flexibility.”

Leerink Partners analyst David Risinger wrote that the big questions remaining are who will replace Marks and whether they will be able to stand up to the Trump administration if needed. In addition, sectors other than vaccines and biotech will likely also be impacted.

“We believe that higher-multiple biopharma stocks are at greater risk than lower-multiple stocks to potential downward pressure,” Risinger wrote. “We note that emerging therapeutic modalities — and especially cell & gene therapy companies — are particularly exposed to FDA disruption risks, given the need for support of science-based novel product development, and their inclusion under CBER authority.”