In the latest pitfall of California’s COVID-19 vaccine rollout, the state’s top epidemiologist recommended Sunday night that clinics put hundreds of thousands of doses on hold after a series of allergic reactions in Southern California.

An abnormally high number of people experienced anaphylactic shock, a severe allergic reaction that requires immediate medical attention, after receiving a shot of the Moderna vaccine at one San Diego vaccination site, Dr. Erica Pan said in statement Sunday night. While the number was fewer than 10, the cluster of negative reactions prompted the California Department of Public Health to recommend pausing the administration of some 330,000 doses from the batch, which had been distributed throughout the state, until an investigation was complete.

Santa Clara County had received 21,800 shots from the batch, which was identified as Lot 41L20A, and shortly after the state issued its recommendation, the county responded by instructing local providers to also halt usage of its allotment from that batch. None of the doses had yet to be administered within the county, officials said. Additionally, they said, there has been no evidence of increased incidents of adverse reactions in Santa Clara County. In Alameda County, health officials said some local providers had received doses from the batch but that there also had been no unusual responses to the vaccine reported to county health.

“Our goal is to provide the COVID vaccine safely, swiftly and equitably,” Pan said in the statement. “A higher-than-usual number of possible allergic reactions were reported with a specific lot of Moderna vaccine administered at one community vaccination clinic. … Out of an extreme abundance of caution and also recognizing the extremely limited supply of vaccine, we are recommending that providers use other available vaccine inventory and pause the administration of vaccines from Moderna Lot 041L20A until the investigation by the CDC, FDA, Moderna and the state is complete. We will provide an update as we learn more.”

The 330,000 doses in the batch from Moderna amount to just below 10% of all the vaccine doses allocated to California thus far. But of the 3.5 million doses the state has received, according to data from the Centers for Disease Control and Prevention, it has administered fewer than one-third of those, just over 1 million doses. On a per-capita basis, only five states have delivered fewer shots into arms than California, according to the CDC data.

In Santa Clara County, the providers who received vaccine doses from the potentially problematic batch include Santa Clara Valley Medical Center, Stanford Health Care and El Camino Health. County health officials said they are working closely with the state and federal agencies and will provide updates when more information is available.

The county has outpaced the state in administration of its vaccinations. Of the approximately 153,000 doses it has received, two-thirds have made their way into residents’ arms, according to the county’s vaccine dashboard. But the county will now shelve about 14% of the doses it has received thus far — an even larger share of those that haven’t yet been administered — because of the adverse effects at the single Southern California clinic.

The reactions reported at the San Diego clinic were similar to rare occurrences that the CDC had warned of and cautioned to be on the lookout for. In California, vaccination sites have staff who monitor recipients on-site for 15 minutes after receiving the shot for adverse reactions. The cluster in Southern California was the first in the nation concerning enough to pause usage altogether.

According to KGTV in San Diego, six recipients of the vaccine at a vaccination “super station” experienced adverse reactions last week, which officials also described as an abnormally high amount, prompting the site to switch to another batch of vaccines.

Severe allergic reactions, while possible, are believed to be exceedingly rare in the Moderna and Pfizer vaccines. The rate of anaphylaxis in the Moderna vaccine was expected to be about one in every 100,000, officials said. Of 1.9 million first doses of the Pfizer vaccine studied by the CDC, there were only 21 cases of anaphylaxis observed, in a report published earlier this month.

Multiple federal and state agencies had opened an investigation into the cases and said they would have more information later this week.