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Patients who experienced allergic or potentially allergic reactions to their first dose of mRNA vaccines for COVID-19 were able to tolerate the second dose, a retrospective study found.

Among 189 patients who had immediate reactions after their first dose of either Pfizer or Moderna's vaccine, all 159 who received a second dose tolerated it, including 19 individuals with first-dose anaphylaxis, reported Kimberly G. Blumenthal, MD, of Massachusetts General Hospital in Boston, and colleagues.

Only 20% of participants had immediate or potential allergic reactions to the second dose, which were either mild or easily resolved by antihistamines, the group wrote in .

"Our take-home message is that immediate allergic symptoms after mRNA vaccines ... do not preclude their future use," co-author Matthew Krantz, MD, of Vanderbilt University in Nashville, told MedPage Today. "Complete two-dose vaccination has become even more important with the Delta variant, and we suspect there are many more people who did not get their second shot because of allergic symptoms."

Although uncommon, up to 2% of all recipients of Pfizer or Moderna's COVID-19 vaccines experience an allergic reaction. Anaphylaxis affects approximately 2.5 out of every 10,000 mRNA COVID-19 vaccine recipients, the authors said. The CDC recommends a 30-minute observation period for vaccine recipients with a history of severe allergic reactions and a 15-minute observation period for those with a history of allergic reactions.

Prior research in recipients with a history of hypersensitivity to their first mRNA dose demonstrated that using a graded dosing protocol was successful in the safe administration of the second dose.

For their study, Blumenthal's group assessed the second-dose safety of the Pfizer-BioNTech or Moderna vaccine in 189 people who previously reported allergic reactions to the first dose (mean age 43; 86% women). In all, 84% received a second dose. As a precautionary measure, 30% of participants were administered an antihistamine medication prior to receiving their second vaccine dose.

The study period was from January to March 2021 and included five U.S. centers: Massachusetts General Hospital; Brigham and Women's Hospital in Boston, Vanderbilt University Medical Center in Nashville, Tennessee; Yale School of Medicine in New Haven, Connecticut; and the University of Texas Southwestern Medical Center in Dallas.

The primary outcome assessed patient tolerance to the second COVID-19 vaccine dose as characterized by no immediate symptoms after administration or mild symptoms solely resolved by antihistamines. Immediate vaccine allergic reactions after the first dose were defined by the presentation of at least one symptom within 4 hours of the first dose administration, resulting in a patient referral to attend a clinical allergy consultation or telehealth appointment for further assessment.

"Second dose tolerance following reactions to the first dose argues that either many of these initial reactions are not all truly allergic reactions, or supports an allergic, but non-immunoglobulin E-mediated mechanism in which symptoms can typically be abated with pre-medications," wrote Blumenthal and coauthors.

Common allergic reactions to the first dose of mRNA vaccine included erythema (28%), dizziness (26%), tingling (24%), tightness in the throat (22%), or the development of hives (21%). Thirty-two patients met anaphylaxis criteria.

"It is true that anaphylaxis with dose two would be possible even though we did not observe it in our study," Krantz told MedPage Today. "We recommend that all patients with immediate and potentially allergic reactions to dose one discuss a safe plan for dose two with their doctor."

Krantz said that plan could involve giving a different vaccine type or having the second dose administered in a medical facility.

CDC previously said that recipients of mRNA vaccine who experienced immediate allergic reactions could be permitted to receive a subsequent dose of Johnson & Johnson's COVID-19 vaccine. But the authors stated that, given their findings, "it may not be necessary to consider this, to our knowledge, largely unstudied alternative mixed series approach."

In addition to its retrospective design, the authors acknowledged several limitations to this study, including the possibility for referral bias to have skewed eligibility and reported allergic reaction severity.

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