Moderna (NASDAQ:MRNA) said on Tuesday that it was investigating a few cases in which patients appeared to have allergic reactions to its coronavirus vaccine. All of the reported incidents, in which people experienced reactions following their first jab of mRNA-1273, occurred at a vaccination clinic in San Diego. (The vaccine is administered in a two-dose regimen, with the shots given 21 days apart.)

The California Department of Health did not specify the exact number of people in the cluster who experienced those reactions, but California State Epidemiologist Dr. Erica S. Pan said it was fewer than 10 individuals over a period of 24 hours.

In each of those cases, the vaccine came from one lot -- No. 41L20A, which comprised over 1.27 million doses that have been distributed to around 1,700 vaccination sites across 37 states. More than 330,000 of those went to California. It is unknown precisely how many of those have already been used. So far, no similar clusters of allergic reactions have been reported.

A syringe being filled with a vaccine from a vial.

Image source: Getty Images.

Following the reports regarding the San Diego cluster, on Sunday, Dr. Pan -- in a decision she said she made "[o]ut of an extreme abundance of caution" -- issued a formal recommendation that health providers immediately pause using vaccines from the lot in question.  Pan promised that she would provide an update as her department learned more about the incidents.

As for Moderna, the company said it is working with federal health officials to investigate the possible causes of the reactions in California. It added that it is unaware of similar clusters of incidents elsewhere.

Thus far, mRNA-1273 remains one of only two coronavirus vaccines to have received emergency use authorization from the Food and Drug Administration. Like the other, Pfizer and BioNTech's BNT162b2, it is being distributed throughout the country and the available supply is being administered to the populations most at risk from COVID-19.

The vaccine rollout process has been slower than expected, however, and incoming President Joe Biden has pledged to speed up the pace in the coming weeks and months.

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