Serious allergic reactions after  COVID-19 vaccines are likely more common than serious reactions following flu vaccines, but are still very rare, according to a new report.

The report authors, from the Centers for Disease Control and Prevention (CDC), analyzed data from the first 1.9 million doses of COVID-19 vaccine administered in the U.S. from Dec. 14 through Dec. 23. For most of this time, only the Pfizer-BioNTech COVID-19 vaccine was available in the U.S. 

The researchers identified 21 cases of people who experienced anaphylaxis — a potentially life-threatening allergic reaction — shortly after receiving the COVID-19 vaccine. 

That's a rate of about 11 cases of anaphylaxis per 1 million doses of vaccine administered. For comparison, the rate of anaphylaxis following flu vaccination is 1.3 cases per million people, Dr. Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, said during a news conference on Wednesday (Jan. 6).

Even though the rate of anaphylaxis tied to COVID-19 vaccines is about 10 times higher than what's seen with flu vaccines, "it's still exceedingly rare," Messonnier said. Overall, the benefits of the vaccine outweigh the potential risks, she said.

Related: 7 strange signs you're having an allergic reaction

Of the 21 cases described in the report, 17 had a history of allergies or allergic reactions, including reactions to drugs, food and insect stings; and seven of those people had experienced anaphylaxis in the past. Nearly all of these patients were treated with epinephrine following their reaction to the COVID-19 vaccine, and all 21 recovered.

Although the new report included only cases of Pfizer COVID-19 vaccine, the CDC is aware of confirmed cases of anaphylaxis tied to the Moderna COVID-19 vaccine as well.

Exactly what's causing these reactions is unclear, and "tremendous efforts" are underway to better understand the cause, Messonnier said. One potential culprit is polyethylene glycol, an ingredient in both the Pfizer and Moderna vaccines, Live Science previously reported. 

However, having allergies doesn't necessarily put you at higher risk of reactions to COVID-19 vaccines. Allergies are common in the U.S., but serious reactions to the vaccines are not.

The CDC recommends that people with a history of reactions to vaccines or anaphylaxis due to any cause should be observed by health care staff for 30 minutes after they receive their COVID-19 shot. (In the report, patients experienced anaphylaxis on average about 13 minutes after their COVID-19 shot.)

The agency recommends that people not receive the Pfizer or Moderna vaccine if they have a known allergy to an ingredient in these vaccines, including polyethylene glycol; and people should not receive a second dose if they have a serious reaction to the first.

Originally published on Live Science.