About one in 100,000 people who received the Pfizer-BioNTech COVID-19 vaccine have had serious allergic reactions, US health officials said Wednesday, emphasizing that the benefits of immunization far outweigh the known risks.
The data comes from the Centers for Disease Control and Prevention (CDC), which documented 21 cases of anaphylaxis following administration of a reported 1,893,360 shots from Dec. 14 to Dec. 23.
“This comes out to an average of 11.1 cases of anaphylaxis per million doses administered,” senior CDC official Nancy Messonnier told reporters.
In comparison, influenza vaccines cause about 1.3 cases of anaphylaxis per million doses administered, so the rate of anaphylaxis for the Pfizer vaccine is about ten times higher.
Messonnier added that cases of anaphylaxis were still “extraordinarily rare” and that it remains in the interest of people to take the vaccine, especially in the context of the COVID-19 pandemic that poses a much greater risk to their health forms.
“A valuable proposition for someone to get vaccinated is the risk of COVID, and poor results from COVID still outweigh the risk of a serious vaccine outcome,” she said.
“Fortunately, we know how to treat anaphylaxis, and we have made provisions to make sure that the people administering the vaccine at immunization sites are ready to treat anaphylaxis.”
The 21 cases ranged in age from 27 to 60 years old, with a median age of 40 years, and all but two were treated with epinephrine.
Nineteen of the cases (90 percent) occurred in women and the median onset time of symptoms was 13 minutes, but ranged from 2 to 150 minutes.
Four (19 percent) of patients were hospitalized, including three in the intensive care unit, and 17 (81 percent) were treated in an emergency department. All but one were known to have been fired from home or recovered at the time of the investigation, and there were no deaths.
Symptoms included skin rash, closed throat sensation, swollen tongue, hives, difficulty breathing, hoarseness, swollen lips, nausea and persistent dry cough.
Research is ongoing
The US has approved two vaccines for emergency use so far: one developed by Pfizer and the other by Moderna.
Both are based on the very latest mRNA (messenger ribonucleic acid) technology and authorities have affixed similar warning labels to both, advising that people with known history of allergic reactions to the vaccine ingredients should not use them.
People who respond severely to the first dose are also asked not to take a second dose.
Messonnier said research was underway to determine what could be causing the allergies.
There is not yet enough data to know the anaphylaxis rate for the Moderna vaccine, which was approved in the US one week after the Pfizer injection, and whether a significant difference will develop between the two vaccines.
A tentative hypothesis for the reactions is the presence of the compound polyethylene glycol (PEG), which has never been used in an approved vaccine before but is found in everyday products including laxatives, shampoos and toothpastes.
Both the Pfizer and Moderna vaccines use PEG molecules as part of the protective shell around their main ingredient, the mRNA that sends genetic instructions to cells.
© Agence France-Presse