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USF Will Study Allergic Reactions To COVID-19 Vaccines

Severe allergic reactions to the Moderna and Pfizer shots have been rare but "higher than what we see with typical vaccines,,” says Dr. Tom Casale, an allergy and infectious disease expert at USF.

The University of South Florida is participating in a clinical trial to determine whether people with a history of severe allergies are at greater risk for allergic reactions to the Moderna or Pfizer coronavirus vaccines.

As of Tuesday, 132,321,628 people have received at least one dose of either vaccine in the United States.

Of those who received the Pfizer shot, 10 to 12 people per million experienced anaphylaxis. With Moderna, that number fell to less than 3 per million.

“These incidences have been rare, but are still higher than what we see with typical vaccines, such as those for influenza,” said Dr. Tom Casale, professor of medicine and pediatrics at USF and director of clinical research in the division of allergy immunology. “With vaccines in general, severe allergic reactions occur at a rate of about 1.3 people per million injections.”

People who have a history of severe allergies — whether to certain drugs, foods, insects or other precipitators — were more likely to have acute allergic reactions to the vaccines.

But those with food allergies should not be worried, Casale said. Unlike vaccines incubated in eggs, such as those used to prevent influenza, the Pfizer and Moderna shots do not contain any food residue.

Researchers have hypothesized that the compounds added to the vaccine to improve its solubility, keep it sterile and stimulate an immune response could be eliciting these reactions.

“We don't think it's the mRNA,” Casale said. “We think that one of the excipients could be responsible for these reactions. That's one of the things that we will be looking at very closely.”

Sponsored by the National Institutes of Health, the study aims to enroll 3,400 participants between the ages of 18 and 69 across 29 sites. About two-thirds of the participants will have severe allergies. The other third will have no history of allergies.

Participants will be divided into groups and assigned at random to receive either the Pfizer or Moderna vaccine, a placebo followed by the Pfizer vaccine, or a placebo followed by the Moderna vaccine. Those who initially receive the placebo vaccine will later return to get a dose of the actual vaccine.

They will also be closely monitored for at least 90 minutes after each injection, more than the 30-minute on-site observation period recommended for people with any history of allergic reactions by the Centers for Disease Control and Prevention.

“The advantage of getting into a study like this is that you are under observation of several physicians the whole time, and you're there for at least 90 minutes after the vaccine,” Casale said. “So if there was an acute allergic reaction, it would be recognized and treated very rapidly.”

After being treated, those who have a reaction will be studied through blood work and genetic testing.

Researchers will determine whether affected participants release chemicals that are typical of an allergic reaction and then examine the potential mechanisms involved in inducing that reaction to better understand why it occurs.

“The importance of doing this is that not only will you define what's going on right now, but it's not unreasonable to expect that we may have to get booster vaccinations with these mRNA vaccines,” Casale said. “And if we could define a way to make them safer when you get your booster, that would be important.”

Though there is a slight risk with all vaccines, Casale said the benefits of being vaccinated against COVID-19 far outweigh the risks from catching the disease.

"Anybody that has vaccine hesitancy, think about the fact that you have almost 600,000 people that have died in the United States alone from the virus versus one of these very rare events, most of which can be effectively recognized and treated,” Casale said.

USF is still looking for participants who have a history of severe allergic disorders and have not yet been vaccinated. Interested individuals should call 813-631-4024 or email

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Jacob Wentz