California’s state epidemiologist is urging a halt to more than 300,000 coronavirus vaccine doses by Moderna because some people who received it needed medical treatment for possible severe allergic reactions.

Dr. Erica S. Pan is recommending that vaccine providers stop using one lot of the Moderna vaccine pending completion of an investigation. She says less than 10 people who were inoculated at a single vaccination site needed medical attention.

But she also said serious reactions to vaccinations are extremely rare.

The virus has claimed more than 33,000 lives in California.