Shares in AstraZeneca PLC (LSE:AZN) and GSK PLC (LSE:GSK, NYSE:GSK) on the back of an appointment by Donald Trump and his health secretary Robert F Kennedy Jnr of a key position at the US drug regulator.
Vinay Prasad, a vaccine sceptic like Kennedy, has been made head of the US Food and Drug Administration's Center for Biologics Evaluation and Research.
The news resulted in 10-30% falls for smaller US biopharma names overnight, while giants such as Eli Lilly, AbbVie, Merck & Co, Amgen, Pfizer Gilead, Bristol Myers Squibb were all down between 3% and 5%.
Prasad's appointment "could be negative for vaccine makers", said Saxo market analyst Wilson. "He's there to tighten up vaccine approvals - it could be much longer approval timelines for new drugs."
A warning from Danish drug giant Novo Nordisk also heaped pressure on the sector, other analysts said.