Image of Kimberly G. Blumenthal, M.D., MSc

Most patients with immediate and potentially allergic reactions to COVID-19 vaccines from Pfizer-BioNTech and Moderna can safely tolerate a second dose, according to a multisite study.

Investigators analyzed data from patients who sought specialist hospital care after reacting to their first mRNA COVID-19 vaccine dose. “These reactions could include symptoms such as itching or hives or flushing,” reported study co-author Matthew S. Krantz, M.D., of Vanderbilt University Medical Center.

Fully 17% of the 189 study participants experienced anaphylaxis — an acute reaction — after the first dose. Among the 159 who received a second dose, 20% had reactions that were mild or resolved by antihistamines. But all of these individuals — including 19 who had initially experienced anaphylaxis — safely completed vaccination. What’s more, all were cleared to receive an mRNA vaccine shot in the future when needed, reported Krantz and colleagues.

When people can tolerate a second vaccine dose after a reaction to the first, it may mean one of two things, the investigators said: Either the reaction is not a truly allergic reactions, or the allergic response is one that can be lessened by taking an antihistamine before getting the shots, they said.

“For classic allergy, re-exposure to the allergen causes the same or even worse symptoms,” said co-author Kimberly G. Blumenthal, M.D., MSc, of Massachusetts General Hospital. The study results show that it is safe for most people to get a second shot, and that it may be helpful for some to consult with an allergy specialist to help guide vaccine decisions, Blumenthal and her colleagues said.

The Centers for Disease Control and Prevention has recommended that people who have a reaction to the Pfizer or Moderna vaccines take the Johnson & Johnson vaccine as a second shot (the J&J vaccine is made using a different technology than mRNA).  But the new study suggests that this “mixed” series of shots may be unnecessary, the authors concluded. 

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Vaccine antibody levels start to wane at around 2 to 3 months The total antibody levels from COVID-19 vaccination may decline starting as early as six weeks after full vaccination, according to a new study of the Pfizer and AstraZeneca vaccines. Levels may then fall by more than 50% over the subsequent 10-week period, investigators from University College London found. The clinical implications of the study, which followed 600 participants, are not yet clear. But other research shows that the vaccines remain effective against severe disease months after shots are received. Notably, antibody levels were substantially higher following two doses of the Pfizer vaccine than after two doses of the AstraZeneca vaccine. They also were relatively higher in recipients with prior SARS-CoV-2 infection.