Covid-19: One in 100,000 people have allergic shock to Pfizer vaccine, according to US study



One in about 100,000 people has experienced a severe allergic reaction after receiving a dose of Pfizer-BioNTech’s vaccine against Covid-19, US health officials said on Wednesday January 6. The latter underline, despite these results, that the benefits of vaccination far outweigh the potential risks.

For the coronavirus, the figure was calculated by the Centers for Disease Prevention and Control (CDC), which identified 21 cases of anaphylactic shock, a severe allergic reaction, out of a total of 1,893,360 injections of the vaccine given between December 14 and 23, 2020. “On average, this represents a rate of 11.1 anaphylactic shocks per million doses administered”, has explained (link in English) to journalists Nancy Messonnier, a CDC official.

For comparison, influenza vaccines cause about 1.3 anaphylactic shock 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 “excessively rare” and the population has every interest in being vaccinated, the risks of the Covid-19 pandemic being much greater, insists Nancy Messonnier. “We know how to treat anaphylactic shock and we have implemented procedures at the vaccination sites” to react when needed, 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. Nineteen cases (90%) of allergic shocks occurred in 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.

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 has never yet been used in vaccines.

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 has never been approved in a vaccine until now, and contain PEG. There are still insufficient data to determine the rate of anaphylactic shock after injection of Moderna vaccine.

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