July 27, 2021 -- People who experienced an allergic reaction to a first dose of either the Pfizer or Moderna COVID-19 vaccine safely tolerated a second dose in a new study.

The findings, however, contrast with official recommendations on management of an allergic reaction to a first vaccine dose.

"Our take home message is that immediate allergic symptoms after mRNA vaccines from Pfizer or Moderna do not preclude their future use," senior author Kimberly Blumenthal, MD, says. Blumenthal is an allergist/immunologist and drug allergy researcher at Massachusetts General Hospital and assistant professor of medicine at Harvard Medical School in Boston.

The results were published July 26 as a research letter in JAMA Internal Medicine.

A total of 32 people in the study, or 17% of 189 with initial allergic reactions, met criteria for anaphylaxis, a more severe form of allergic reaction. Flushing, dizziness or lightheadedness, tingling, throat tightness, hives, and wheezing or shortness of breath were additional first dose reactions reported.

Out of 189 people with initial allergic reactions, investigators looked at 159 or 84% who completed a two-dose regimen. All patients in this group, including 19 individuals with first-dose anaphylaxis, tolerated the second dose, Blumenthal, lead author Matthew Krantz, MD, and colleagues reported.

Overall, 32 individuals with who received a second dose reported immediate and potentially allergic symptoms. "While 20% had symptoms with dose two, the symptoms were manageable and not anaphylactic," Blumenthal said.

"However, it is true that anaphylaxis with dose two would be possible even though we did not observe it in our study," she said.

The average age of the 189 people in the study was 43 years and 86% were women. Allergic vaccine reactions were reported in the first 3 months of 2021.

"While the study is compelling, the numbers are small and there is uncertainty regarding true allergic risk in patients who had anaphylaxis with the first dose of mRNA vaccine. Nineteen of 32 patients with anaphylaxis safely received a second dose, but it is not known what would have happened with the remaining 13 patients," Niraj Patel, MD, says.

"More data is needed, but the results look encouraging," added Patel, chair of the American College of Allergy, Asthma, and Immunology COVID19 Vaccine Task Force.

"This study confirms that the vaccine is safe for re-administration, under the supervision of an allergist, in cases where there was a suspected first dose reaction," Matthew Greenhawt, MD, allergist and immunologist and director of the Food Challenge and Research Unit at Children’s Hospital Colorado in Aurora, said in an email.

Greenhawt and colleagues authored a June 2021 review and meta-analysis evaluating risk of allergic reactions to COVID-19 vaccines. The current results are consistent with their findings "where the evidence had suggested these reactions were very rare – though still may occur – and without any clear evidence of a provoking allergen within the vaccine."

Have a Plan

For this reason, Blumenthal suggested all patients who experience an immediate and potentially allergic reaction to a first vaccine dose discuss a safe plan to receive the second dose with their doctor.

"While we still do not know the allergic component of mRNA vaccines, this study suggests that some patients may tolerate a second dose of the same mRNA vaccine after having an initial allergic reaction," Patel said.

"This is important because in certain patients who had anaphylaxis with the first dose, current recommendations is not to receive a second dose of mRNA vaccine, but rather a non-mRNA vaccine such as [/Johnson&Johnson] as an alternative second dose," Patel said.

He cautioned that the efficacy of such mixed vaccine regimens has not been fully studied.

The CDC, for example, recommends against a second dose of the same mRNA vaccine following a severe initial allergic reaction, one strong enough to require epinephrine or hospitalization. Also, the agency suggests people who have an immediate but less severe allergic reaction should ask their doctor about switching to a non-mRNA vaccine for their second shot.

In addition to switching vaccine types, Blumenthal said getting dose two of an mRNA vaccine in a medical facility might be another option.

Blumenthal and colleagues noted that some initial reactions may not have been truly allergic.

"This is a subtle but very important point, which underscores multiple other aspects of how some have recommended to evaluate these reactions or persons felt to possibly be at risk for a reaction to this class of vaccine," Greenhawt said.

More Urgency With Delta Variant?

Complete two-dose vaccination "has become even more important with the Delta variant. We suspect there are many more people who did not get their second shot because of allergic symptoms," said study co-author Aleena Banerji, MD, clinical director of the Allergy and Clinical Immunology Unit at Mass General and an associate professor at Harvard Medical School.