A U.S. health agency on Wednesday announced the launch of a study of allergic reactions to two COVID-19 vaccines.

The phase two trial will examine whether people who are highly allergic or have a mast cell disorder are at increased risk for an immediate, systemic allergic reaction to the vaccines made by Moderna and Pfizer-BioNTech.

A systemic allergic reaction to a vaccine means a reaction occurs in one of more parts of the body beyond the site of the injection.

If such reactions occur during the study, investigators will assess whether they are more frequent in participants who have a history of allergies or have mast cell disorder, versus other trial participants. Investigators will also probe the biological mechanism behind the reactions and try to pinpoint whether genetic patterns or other factors could predict which people are most at risk.

The trial is being funded and sponsored by the National Institute of Allergy and Infectious Diseases (NIAID), which is part of the National Institutes of Health.

“The public understandably has been concerned about reports of rare, severe allergic reactions to the Moderna and Pfizer-BioNTech COVID-19 vaccines,” Dr. Anthony Fauci, the NIAID director, said in a statement.

“The information gathered during this trial will help doctors advise people who are highly allergic or have a mast cell disorder about the risks and benefits of receiving these two vaccines. However, for most people, the benefits of COVID-19 vaccination far outweigh the risks,” he added.

The trial will enroll 3,400 adults between the ages of 18 and 69 at 35 allergy-research centers across the nation. About six in 10 of the participants must have either a history of severe allergic reactions or a diagnosis of a mast cell disorder. The others must not.

Approximately two-thirds of volunteers will be female, since the vast majority of cases of anaphylaxis to COVID-19 vaccines have occurred among women.

Participants will be randomly assigned to receive a vaccine or a placebo followed by a vaccine. Every participant will ultimately get two doses of a vaccine.

Results are expected in the late summer.

Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, is seen before receiving his first dose of the Moderna COVID-19 vaccine at the National Institutes of Health, in Bethesda, Md., on Dec. 22, 2020. (Patrick Semansky/Pool via Reuters)

The agency did not immediately respond to an inquiry asking why it has taken so long to launch a trial exploring allergic reactions to vaccines. Fauci and other health officials were not asked about and did not mention the trial during a virtual press briefing on Wednesday morning.

Both Moderna and Pfizer received emergency use approval for the COVID-19 shots in December 2020.

More than 168 million doses have been administered in the United States as of April 6. All but 4.3 million are from Moderna or Pfizer.

An early count of severe allergic reactions among those getting a COVID-19 vaccine showed the rate was much higher than those who receive flu vaccines, 11.1 people per million vaccinated versus 1.3 per million.

According to U.S. officials, the allergic reaction rate post-vaccination has since dropped to approximately 2 to 5 million. The Centers for Disease Control and Prevention (CDC) and Food and Drug Administration describe anaphylaxis as “rare” following a COVID-19 shot.

In a small percentage of cases, the patient has died.

Health officials say that a review of available clinical information including death certificates, autopsy, and medical records revealed no evidence that vaccination contributed to patient deaths.

Facilities administering COVID-19 vaccines should have at least three doses of epinephrine available at all times and should observe people getting vaccinated for at least 15 minutes, according to the CDC. Those with a history of allergic reactions to vaccines or a history of anaphylaxis due to any cause should be observed for at least 30 minutes.

Signs of severe allergic reactions include throat tightness, nausea, dizziness, agitation, and a sudden increase in secretions from the eyes, nose, or mouth.

Symptoms of anaphylaxis often occur within 15-30 minutes of vaccination, though it can sometimes take several hours for symptoms to appear.

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