San Diego, California - California's top epidemiologist told healthcare providers on Sunday to stop using a batch of Moderna's Covid-19 vaccine after a "higher than usual" number of people had apparent allergic reactions at a San Diego vaccination clinic.

Erica Pan, the state epidemiologist, said Sunday that "out of an extreme abundance of caution," providers should stop using the doses until federal, state and company officials finish an investigation.

California has received about 330,000 doses from the Moderna lot in question – about 10 per cent of all the vaccine, both Moderna's and Pfizer's, that have been distributed across the Golden State, officials said.

The apparent allergic reactions occurred at San Diego County's drive-through mass vaccination site at Petco Park, said California Department of Public Health spokesman Darrel Ng.

A "higher than usual" number of patients who received the vaccine appeared to be suffering from allergic reactions. Their symptoms were severe and required medical attention, Pan said.

Eric McDonald, San Diego County's director of epidemiology, said during a livestreamed news briefing Wednesday that operations at Petco Park slowed down after six healthcare workers who had just been vaccinated had apparent allergic reactions.

"That number, clustered together, was slightly higher than expected for the time period," McDonald said. He said the health providers swapped out the batch of vaccines they were administering, "in the possible event" that reactions were connected.

"Any vaccination, whether it's the Covid vaccination or the flu vaccination at your doctor's office, should be given under medical supervision," McDonald said. "In this case, with a new vaccine, there are very specific guidelines that all sites giving this vaccine must follow."

That includes monitoring patients after the shot is administered for at least 15 minutes, and longer in cases where patients have a medical condition that could lead to an allergic reaction.

The Petco Park site has the capacity to administer 5,000 shots per day. It's one of several mega-sites that launched in California within the last week, including a similar clinic at Dodger Stadium that city officials say will soon have a capacity of about 12,000 shots daily.

No other cluster of issues, or individual issues, have been reported with the Moderna batch, lot 041L20A, which was distributed to 287 medical providers across the state, officials said. The batch arrived in California between January 5 and January 12.

"There are not immediate replacement doses during the pause in addition to pending orders scheduled for shipment in the next few days," the state said in a statement sent to medical providers.

Moderna did not immediately return a request seeking comment.