Allergic reactions to COVID-19 vaccine remain ‘overwhelmingly rare,’ says expert

A healthcare worker gives the COVID-19 vaccine to Kassandra Martinez, an EVS Attendant and Lead, at Rady Children's Hospital in San Diego, California on December 15, 2020. (Photo: ARIANA DREHSLER AFP)
A health care worker gives the COVID-19 vaccine to an employee at Rady Children's Hospital in San Diego. A cluster of allergic reactions at another facility in the area has prompted officials to halt the distribution of a batch of the Moderna vaccine. (Photo: Ariana Drehsler/AFP)

The California Department of Public Health (CDPH) has halted the distribution of a batch of the Moderna COVID-19 vaccine — lot 41L20A — after multiple individuals experienced severe allergic reactions at a single clinic in less than 24 hours. But speaking to Yahoo Life, Dr. Purvi Parikh, a member of the American College of Allergy, Asthma and Immunology COVID-19 Vaccine Task Force, said the investigation that state officials have launched is “normal and necessary.”

“If they are all true allergic reactions, then it may be a higher frequency than usual, but we need to be sure these were true allergic reactions, as many other events mimic allergic reactions,” Parikh told Yahoo Life. “We need to investigate what actually caused them.” In a statement released over the weekend, California state epidemiologist Dr. Erica S. Pan said the move to stop distributing vaccines from that batch was done out of “an abundance of caution.”

“Our goal is to provide the COVID vaccine safely, swiftly and equitably,” Pan noted. “A higher-than-usual number of possible allergic reactions were reported with a specific lot of Moderna vaccine administered at one community vaccination clinic. Fewer than 10 individuals required medical attention over the span of 24 hours.” Pan said the reactions were consistent with previous reports from the Centers for Disease Control and Prevention, and were identified during the “standard observation period” (15 or 30 minutes, depending on whether the individual has a history of allergic reactions).

The CDPH release noted that all subjects “appeared to be experiencing a possible severe allergic reaction,” defined as a type of adverse event that the CDC reports some people have experienced when receiving a COVID-19 vaccination, and that “the vaccine site switched to another lot of Moderna vaccine after closing for a few hours.”

Allergic reactions have been reported in response to both Moderna’s and Pfizer’s vaccines but are uncommon. This month the CDC released a report on the first 1.9 million people to get vaccinated for COVID-19 and found that approximately 1 in 100,000 individuals had a severe allergic reaction to the Pfizer vaccine. The organization found anaphylaxis, a potentially life-threatening reaction, to be similarly uncommon, concluding that the chances of it occurring are 11 in 1 million.

As of now, it remains unclear what may be causing the reactions and how this specific batch of the Moderna vaccine may have contributed. But some experts have speculated that the responses may be related to a compound present in both Moderna’s and Pfizer’s vaccines known as polyethylene glycol, or PEG. Parikh said that although PEG could be the source, experts “do not know definitively” at this point.

“It seems to be the most likely cause ... as the other ingredients, such as sugars, lipids and salts, aren’t very allergenic,” Parikh explained. “Overall, polyethylene glycol allergy is pretty rare, as it is a commonly used ingredient in many medications such as Miralax or Toradol, as well as in other common products like toothpaste.”

In a statement emailed to Yahoo Life, Moderna acknowledged that it has received a report about multiple allergic reactions at the facility in San Diego, but added that there is no reason to believe there will be more. “At this point, Moderna is unaware of comparable clusters of adverse events from other vaccination centers which may have administered vaccines from the same lot, or from other Moderna lots,” the pharmaceutical company said.

Moderna said the batch includes more than 1.2 million doses of the vaccine, delivered to 1,700 vaccination sites in 37 states. More than 330,000 of those doses were sent to California and distributed to 287 providers, but the vast majority remain unused. The CDPH was not able to share a timeline of when the doses will be cleared, but Parikh said it shouldn’t change anyone’s vaccination plans.

“Americans should not be hesitant,” said Parikh. “Vaccine reactions are still overwhelmingly rare. With 400,000 American deaths this year and climbing, the risk of getting COVID-19 and getting seriously ill or dying from it is far more common.”

For the latest coronavirus news and updates, follow along at According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please reference the CDC’s and WHO’s resource guides.

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