Moderna Inc. fell Monday after the Food and Drug Administration said in a memo late last week it would place new restrictions on which vaccines hit the market.

Vinay Prasad, a top FDA official, blamed Covid shots for playing a role in the deaths of 10 children in the memo he sent to agency staff on Friday, which was seen by Bloomberg. He said the FDA will require vaccine makers to perform additional tests to get their shots approved as a result of the safety concerns.

Moderna, which produces Covid shots, fell 5.8% in morning trading. Other vaccine makers also dipped. Vir Biotechnology and Vaxcyte fell 8%; Novavax, Hims & Hers, BioNTech SE, and Biogen fell 3%; Regeneron Pharmaceuticals and Amgen fell 2%; Merck and AstraZeneca fell 1%.

The decision has the potential to curtail which vaccines are available. Prasad said the agency would focus on rethinking its guidelines around flu shots and require companies to present more data, which can be expensive and difficult to produce. The FDA will also tighten its rules around vaccines marketed to pregnant women and demand that pneumonia shot makers prove their vaccines reduce the disease.

The US approach toward vaccinations has shifted under Health and Human Services Secretary Robert F. Kennedy Jr., a longtime vaccine critic. The FDA has already curbed access to Covid shots, limiting approvals to vaccines for older people and those with underlying conditions. Separately, a panel of government vaccine advisers has recommended removing some ingredients in shots long targeted by anti-vaxxers, but considered safe by researchers.

In late November, the US Centers for Disease Control and Prevention updated a page on its website to suggest vaccines may cause autism, rejecting longstanding medical consensus on the topic.

Medical groups and public health experts have slammed HHS moves to limit vaccine access, saying they could put millions of lives at risk.

Moderna is developing a flu vaccine and a combination shot for flu and Covid. The company said recently it plans to file for regulatory approval for its flu shot in the US, Europe, Australia and Canada by January. It’s awaiting guidance from US officials before seeking approval for its combination vaccine.

In a note Monday, William Blair analysts said Prasad’s memo suggests Moderna’s flu vaccine could have “increased safety scrutiny.” They also see “further headwinds” for the company’s Covid business if more regulatory restrictions are imposed.