CDC reports more allergic reactions to Covid-19 vaccines, but cases remain few

Twenty-nine people in the United States have developed anaphylaxis after being vaccinated against Covid-19 since the vaccine rollout began, health officials reported Wednesday, with cases occurring after vaccination using both the Pfizer-BioNTech and the Moderna vaccines.

The Centers for Disease Control and Prevention said at present it looks like anaphylaxis cases are occurring at a rate of about 5.5 per 1 million vaccine doses given, though the agency cautioned that figure may change as the vaccination effort continues.

The allergic reactions do not change CDC’s recommendations on who can be vaccinated against Covid-19, with senior officials stressing that the risk of severe illness and death from the disease still outweighs the risk of developing anaphylaxis after vaccination.

“Of course, we all would hope that any vaccine would have zero adverse events. But even at 11 cases per million doses administered, it’s a very safe vaccine,” said Nancy Messonnier, director of CDC’s National Center for Immunization and Respiratory Diseases. “We’re in the setting of 2,000 Covid deaths per day. … I would say it’s still a good value proposition for someone to get vaccinated.”

Messonnier made her remarks during a press briefing held to explain data in an early assessment of the anaphylaxis cases by CDC scientists. In the article, published Wednesday in CDC’s online journal Morbidity and Mortality Weekly Report, the researchers evaluated 21 cases of anaphylaxis that occurred in the first week and a half of the vaccination rollout. At that point, 1.9 million people had been vaccinated, for a rate of 11.1 cases of anaphylaxis per 1 million doses of vaccine used.

The period studied ran from the beginning of the national vaccination campaign to Dec. 23, just two days after the Moderna vaccine began to be used. To date most of the cases have occurred after receipt of the Pfizer vaccine, but whether that’s because more of that vaccine has been used is not yet clear. Both vaccines are made using messenger RNA.

Anaphylaxis is a known but rare side effect of vaccination, though the rate at which it occurs following Covid-19 vaccination appears to be higher than it is for some vaccines, such as flu shots. If left untreated, the reaction can be fatal in some cases, though none of the people whose cases were disclosed Wednesday died.

Most of the people who developed the reaction were treated with epinephrine, the drug that is in the EpiPens carried by people who have severe allergies.

The CDC recommends that anyone who had an anaphylactic reaction to the first dose of a Covid vaccine should not have the second dose.

Messonnier said there are “tremendous efforts” underway to try to understand why these vaccines are triggering severe reactions in some people. Most, though not all, of the people who developed anaphylaxis had a history of allergies to a variety of things — nuts, shellfish, some drugs, and wasps — and several had previously experienced anaphylaxis. Most also developed the reaction within 15 minutes of having been vaccinated.

The CDC did not give a breakdown of how many cases occurred overall with the Pfizer vaccine vs. the one developed by Moderna.

“At this point we think that it is something that is seen with both vaccines and therefore our recommendations apply to both vaccines,” Messonnier said.

Those recommendations include urging all sites where Covid-19 vaccines are being administered to be ready to handle anaphylaxis cases, including by having epinephrine at the ready. People who have been vaccinated should stay to be monitored for 15 minutes if they have no history of allergy and 30 minutes if they do.

There was no geographic clustering of cases and people who developed anaphylaxis were not all vaccinated from the same batch of vaccine, suggesting whatever is causing the reaction was not limited to a single batch of product.

This story has been updated with a new rate from the CDC of anaphylaxis cases per vaccine doses given.