1 in 100,000 had severe reactions to Pfizer vaccine: US study

The data comes from the Centers for Disease Control and Prevention, which documented 21 cases of anaphylaxis after administration of a reported 1,893,360 shots from December 14 to December 23.

"This averages out to a rate of 11.1 anaphylaxis cases per one million doses administered," senior CDC official Nancy Messonnier told reporters.

By comparison, flu vaccines cause about 1.3 anaphylaxis cases per million doses administered, and so the rate of anaphylaxis for the Pfizer vaccine is roughly ten times greater.

Messonnier added that anaphylaxis cases were still "exceedingly rare" and it remains in people's best interest to take the vaccine, particularly in the context of the Covid-19 pandemic that is a far greater danger to their health.

"A good value proposition for someone to get vaccinated is their risk from Covid and poor outcomes from Covid is still more than the risk of a severe outcome from the vaccine," she said.

"Fortunately, we know how to treat anaphylaxis, and we've put provisions in place to ensure that at immunization sites, the folks administering the vaccine are ready to treat anaphylaxis."

The 21 cases ranged in age from 27 to 60 years old, with a median age of 40, and all but two were treated with epinephrine.

Nineteen of the cases (90 percent) occurred in females, and the median onset time of symptoms was 13 minutes, but ranged from two to 150 minutes.

Four (19 percent) of patients were hospitalized, including three in intensive care, and 17 (81 percent) were treated in an emergency department. All but one was known to have been discharged home or recovered at the time of the study, and there were no deaths.

Symptoms included rash, sensation of throat closure, swollen tongue, hives, difficulty breathing, hoarseness, swollen lips, nausea and persistent dry cough.