Photograph by Hannah Yoon, Redux
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A nurse prepares vials of the Pfizer-BioNTech vaccine for administration. A new study found that the rate of allergic reactions to the vaccine are extremely low.

Photograph by Hannah Yoon, Redux

What the data show about allergic reactions to COVID-19 vaccines

A new report from the CDC reveals how rare severe reactions actually are and what people can do if they are worried about taking the shot.

As vaccination efforts against COVID-19 haltingly ramp up across the United States, the country’s public health officials are confirming that severe allergic reactions to the Pfizer-BioNTech COVID-19 vaccine are rare.

In a study released January 6 by the U.S. Centers for Disease Control and Prevention (CDC), researchers revealed that the risk of anaphylaxis—a severe, potentially life-threatening allergic reaction—from the vaccine is extremely low. Based on data from people who have received the first of the two recommended doses, only about one in every 90,000 people, on average, will experience this adverse reaction. That’s less than 3 percent of the lifetime risk of dying from choking on food.

Technically, the chance of severe allergic reactions from the Pfizer-BioNTech vaccine is about 8.5 times higher than the risk from the seasonal flu vaccine, which has a rate of about one in 769,000. But experts point out it’s still a tiny number: According to the study, 1.89 million people in the U.S. received the first dose of the vaccine between December 14 to December 23, and more than 99.998 percent of them did not experience anaphylaxis.

The news should help people who have concerns about taking the vaccine. According to a National Geographic-Morning Consult poll conducted in December, seven in 10 Americans say they are wary of the potential side effects of any COVID-19 vaccine, with 58 percent of the respondents saying they will get vaccinated, but not until they have seen how the shot impacts other people.

Overall, though, the data reveal that the risk of anaphylaxis is far smaller than the negative outcomes associated with COVID-19 infections, especially among older Americans. By October 2020, the disease had become one of the leading causes of death in the U.S. among people more than 45 years old, outpacing automotive collisions, suicide, homicide, and accidental drug overdoses. To date, the pandemic has killed more than one out of every 1,000 Americans.

“I and my colleagues at CDC and FDA are committed to ensuring that COVID-19 vaccines are safe,” Nancy Messonnier, director of the CDC’s National Center for Immunization and Respiratory Diseases, said in a Wednesday press briefing on the study. “I know I look forward to the day I get to roll up my sleeve and get vaccinated.”

What do we know about the anaphylaxis cases?

Of the 21 cases of severe anaphylaxis associated with the Pfizer-BioNTech vaccine, 19 happened in women, and half were in people between the ages of 40 and 60. (The other half were between 27 and 40 years old.) Seventeen of the cases were in people with a history of allergic reactions to food, medications, other types of vaccines, or bee and wasp stings. Seven had a past history with anaphylaxis.

In 18 of the 21 documented cases, patients started showing symptoms of anaphylaxis within 30 minutes of getting the dose. Nearly all were treated immediately with epinephrine, a common treatment for anaphylaxis and the active ingredient in an Epi-Pen. Only four patients required hospitalization. At least 20 of the 21 patients had either fully recovered by December 23 or had been discharged from the hospital by then.

There’s no geographic clustering of the cases, and they trace back to different lots of the vaccine, so there’s no evidence that contamination of the vaccine was an issue. Research into the cases is still early, and the skew toward women may speak to a genuine biological phenomenon, or it may reflect the fact that more women than men—62 percent of the 1.89 million—received the vaccine during the study period.

Is there any clear difference in risk between the vaccines from Pfizer-BioNTech and Moderna?

At present, no. By raw totals, more cases arose from the Pfizer-BioNTech vaccine than the Moderna vaccine, but CDC officials say that’s likely because the Pfizer-BioNTech version was authorized first and therefore was given to more people during the study period.

In total, 1,893,360 doses of the Pfizer-BioNTech vaccine were administered during the study period, which is about 8.5 times more than the 224,322 Moderna doses given over that same span. What’s more, the rollout of Moderna’s vaccine didn’t start until the last three days of the study period, on December 21. Of those Moderna doses, the CDC was notified of just one case of anaphylaxis and is awaiting more data.

Should I get the vaccine?

In the briefing, Messonnier and Tom Clark, the leader of the CDC’s vaccine evaluation team, stressed that in general, when people are recommended to get the vaccine, they should get it.

To ensure a swift response in case of a rare allergic reaction, people will need to be observed for 15 minutes after getting the vaccine. People with a history of anaphylaxis or immediate allergic reactions will need to be monitored for 30 minutes.

Clark outlined two specific examples of people who should not get the full two-dose Pfizer-BioNTech vaccine regimen: “If you had an immediate reaction to your first [COVID-19 vaccine] dose, don’t get a second, and if you have a known allergy to components in the vaccine, or very closely related compounds, we recommend you not get vaccinated at this time.” People who are unsure whether they may be allergic to a vaccine ingredient should speak to their doctors before vaccination.

Specifically, the CDC cautions that COVID-19 vaccines that use messenger RNA (mRNA)—including the Pfizer BioNTech and Moderna vaccines—also contain polyethylene glycol to shield the mRNA, so people with allergies to the compound shouldn’t get the vaccine. Likewise, people allergic to polysorbates—which chemically resemble polyethylene glycol—shouldn’t get the shot at this time.

In general, the CDC recommends that people with concerns over allergies should consult with their health-care providers.

“There’s a big difference between somebody who had a mild allergic reaction in their childhood, versus somebody who had a severe allergic reaction last week,” Messionner said. “It’s going to be really important to have a clinician help a patient exercise judgment.”