People who experienced an immediate allergic reaction to their first COVID-19 vaccine are unlikely to experience a severe immediate allergic reaction to a second dose, says a study involving McMaster scientists.

Researchers from Canada and the United States assessed 22 studies involving 1,366 patients who received second doses of COVID-19 vaccines after experiencing allergic reactions to first doses.

Only 0.16 per cent of patients monitored experienced an immediate severe reaction to a second dose; more than 99 per cent of individuals tolerated their second shot with no immediate severe response.

“These findings show that a history of immediate reaction — including severe immediate allergic reactions — to a prior SARS-CoV-2 mRNA vaccine does not guarantee another reaction to the next vaccine dose,” says Derek Chu, first author of the study and assistant professor of medicine in the division of allergy and immunology.

“Our data support safe vaccination of these individuals in a setting with professionals who can manage allergic reactions.”

Less than 5 per cent of patients who suffered an anaphylactic reaction after a first dose had the same reaction a second time. Overall, 13 per cent of patients developed non-severe, manageable immediate symptoms.

No deaths were reported among individuals throughout the 22 studies analyzed.

The study was published in JAMA Internal Medicine, and builds on work begun in 2021, examining allergic reactions to first doses of COVID-19 vaccines.

“What these results tell us is that, for individuals with concerns regarding allergic reaction, taking a risk with vaccination is better than risking getting COVID-19,” said Matthew Greenhawt, senior author of the study and a professor at the University of Colorado School of Medicine.

“Our job as allergists is to manage risk. Those concerned about an immediate allergic reaction can receive vaccination doses under an allergist’s supervision, especially those worried about a second dose. Full vaccination is important in mitigating infection.”