One in about 100,000 people has experienced a severe allergic reaction after receiving a dose of Pfizer-BioNTech coronavirus vaccine, US health officials said on January 6, stressing that the benefits of the vaccination far outweigh the potential risks.

The figure was calculated by the Centers for Disease Prevention and Control (CDC). They identified 21 cases of anaphylactic shock, a severe allergic reaction, out of a total of 1,893,360 injections of the vaccine carried out between December 14 and 23.

“On average, this represents a rate of 11.1 anaphylactic shocks per million doses administered,” Nancy Messonnier, a CDC official, told reporters.

Read also: Should we be afraid of COVID vaccines?

An “excessively rare” reaction

For comparison, influenza vaccines cause about 1.3 anaphylactic shocks per million doses.

Even though the rate for Pfizer-BioNTech’s coronavirus vaccine is about ten times higher, the number of cases of severe allergic reactions remains “exceedingly rare” and the population has every interest in being vaccinated, the risks of the pandemic of Covid-19 being much larger, insisted Ms. Messonnier.

To read also: Allergies: how to prevent anaphylactic shock?

A reaction treated quickly

“We know how to treat anaphylactic shock and we have put in place procedures at the vaccination sites” to react if necessary, she added.

The 21 cases identified by the CDC involved people aged 27 to 60, with a median age of 40. All but two were treated with epinephrine, which is commonly used to treat severe allergies.

Symptoms of allergic shock

19 cases (90%) of allergic shock involved women and symptoms occurred between two and 150 minutes after vaccine injection (median time 13 minutes).

These symptoms included rash, hives, a feeling of suffocation, swelling of the tongue, difficulty breathing, swollen lips, nausea, and a persistent dry cough.

Four patients (19%) were hospitalized, including three in intensive care, and the other 17 were treated in an emergency department. No deaths were reported and all but one were well at the time of writing.

Why this reaction?

Ms Messonnier said investigations were underway to determine the cause of the allergic reactions. One of the hypotheses concerns the presence in the vaccine of a substance, polyethylene glycol (PEG), which is used in many common products such as laxatives, shampoos or toothpaste. But PEG has never been used in vaccines before.

Currently, the US authorities have authorized two vaccines, one developed by Pfizer and another produced by Moderna. Both use a new technology, called messenger RNA (mRNA), which had never been approved in a vaccine until now. They also contain PEG.

But when it comes to the effects of the vaccine developed by Moderna, there is still insufficient data to determine the rate of anaphylactic shock after injection.