Vaccines against Pfizer-BioNTech COVID-19 (left) and Moderna COVID-19 (right).

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The National Institutes of Health announced on Wednesday that some people had suffered severe allergic reactions shortly after receiving the Pfizer-BioNTech and Moderna Covid-19 vaccines.

“The public was understandably concerned about reports of rare, severe allergic reactions to the Moderna and Pfizer-BioNTech COVID-19 vaccines,” said Dr. Anthony Fauci, director of the National Institute on Allergies and Infectious Diseases, which is part of the NIH, said in a statement.

“The information gathered during this study will help doctors educate people with high allergies or mast cell disorder about the risks and benefits of receiving these two vaccines. For most people, however, the benefits of vaccinating COVID-19 far outweigh the risks ” he said.

Medical experts say allergic reactions from vaccines are rare but can occur sometimes. Most of the rare, severe allergic reactions to the vaccines have occurred in people with a history of allergies, according to health officials. An immediate allergic reaction usually occurs within 4 hours of vaccination and, according to the Centers for Disease Control and Prevention, may include symptoms such as hives, swelling, and wheezing.

The CDC said in January that allergic reactions occur at the rate of 11.1 per 1 million vaccinations for the Pfizer shot and 2.5 per 1 million for the Moderna shot.

The NIH study will enroll 3,400 adults between the ages of 18 and 69 in up to 35 academic allergy research centers nationwide. Participants will be divided into groups and selected at random to receive either the Pfizer BioNTech vaccine, the Moderna vaccine, a placebo followed by the Pfizer BioNTech vaccine, or a placebo followed by the Moderna vaccine.

Alkis Togias, director of NIAID’s Allergy, Asthma, and Respiratory Biology division, told CNBC in December that US agency researchers became interested in the rare phenomenon after it was reported that some people responded to Pfizer’s vaccine, called anaphylaxis Having been classified as a Serious and Serious Illness, possibly life-threatening allergic reaction.

The CDC has asked health care providers to monitor patients 15 minutes after vaccination and 30 minutes after vaccination.

If someone has a severe allergic reaction after the first dose of a Covid-19 vaccine, the CDC recommends not receiving the second dose of that vaccine, even if the allergic reaction wasn’t severe enough to require emergency care.