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Interim CDC analysis of severe allergic reactions in people who received the Pfizer-BioNTech vaccine

Anaphylaxis is a serious allergic reaction that can become life threatening. Here are key points about anaphylaxis from the U.S. Centers for Disease Control and Prevention (CDC):

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“Early signs and symptoms of anaphylaxis can resemble a mild allergic reaction, and it is often difficult to predict whether initial, mild symptoms will progress to an anaphylactic reaction.” The agency also stated that “symptoms of [anaphylaxis] often occur within 15 to 30 minutes of vaccination, though it can sometimes take several hours for symptoms to appear.”

Signs and symptoms of anaphylaxis can include the following:

Respiratory

  • “sensation of throat closing, stridor (high-pitched sound while breathing), shortness of breath, wheeze, cough”

Gastrointestinal

  • “nausea, vomiting, diarrhea, abdominal pain”

Cardiovascular

  • “dizziness, fainting, tachycardia (abnormally fast heart rate), hypotension (abnormally low blood pressure)”

Skin/mucosal

  • “generalized hives, itching or swelling of lips, face, throat”

The CDC adds that “persons with communication difficulties” should be monitored for the above signs and symptoms, as well as for additional symptoms such as the following:

  • flushing
  • sudden increase in secretions (from eyes, nose or mouth)
  • coughing
  • trouble swallowing
  • agitation
  • acute change in mental status

Allergic reactions to the Pfizer-BioNTech vaccine

The interim CDC analysis is based on the rollout of the first 1,893,360 doses of the vaccine. After assessing reports of adverse events from this data set, the CDC has concluded that there were 21 cases of anaphylaxis. This results in a rate of 4.7 cases per one million doses. This is very rare.

Cases

A brief profile of the 21 people who had severe allergic reactions is as follows:

  • age range – 27 to 60 years
  • 90% were women (this point is revisited later)
  • time to allergic symptoms – some people developed allergic symptoms as quickly as two minutes after their injection while others took as long as 150 minutes.

The distribution of people in relation to the time to appearance of allergic symptoms was as follows:

  • within 2 to 15 minutes – 71%
  • within 15 to 30 minutes – 14%
  • after 30 minutes – 14%

(The total does not equal 100 due to rounding.)

Most (90%) affected people received injections of epinephrine (adrenaline) after their symptoms appeared.

Four people required hospitalization, three of whom needed admission to an intensive care unit. The remaining 17 people were treated in the emergency department of a hospital.

Complete data were available on 20 of the 21 people (98%). All 20 people recovered and were sent home. No one died.

A history of allergic reactions

The CDC researchers found that 17 people (81%) out of the 21 had “a documented history of allergies or allergic reactions, including to drugs or medical products, foods and insect stings; seven patients (33%) had experienced an episode of anaphylaxis in the past, including one after receipt of a rabies vaccine and another after receipt of an influenza A (H1N1) vaccine.”

The CDC did not find a clustering of severe allergic reactions in any particular region.

Non-anaphylactic reactions

The CDC also reported that separately there were 83 cases of allergic reactions not involving anaphylaxis after vaccination. These reactions occurred within the first day of vaccination.

Common symptoms were as follows:

  • itchy skin
  • rash
  • itchy and scratching sensations in the throat
  • mild respiratory symptoms (the agency did not define these)

Patients with these allergic symptoms were between 18 to 65 years old. About 90% of these reactions were also in women.

The onset of non-anaphylactic reactions from the time of vaccination ranged from less than one minute to 20 hours. The distribution of the onset of these symptoms after vaccination was as follows:

  • within 30 minutes – 85%
  • after 30 minutes – 15%

According to the CDC, in 67% of cases of non-anaphylactic reactions “there was a past history of allergies or allergic reactions.”

The CDC advises that “persons with an immediate allergic reaction to the first dose of an mRNA COVID-19 vaccine should not receive additional doses of either of the mRNA COVID-19 vaccines.”

The issue of gender

Previous reports suggest that, in general, more women experience adverse reactions to flu vaccine than men.

In the rollout of the Pfizer-BioNTech vaccine, 64% of the vaccine patients were women. It is therefore plausible that part of the reason for the over-representation of women in cases of allergic reaction to this vaccine arose in part because most people who received the vaccine were women.

More research is needed to better understand the gendered distribution of vaccine-associated allergic reactions. The CDC’s report should be viewed as preliminary and further monitoring and analysis of side effects will be done as more people get the vaccine.

—Sean R. Hosein

REFERENCES:

CDC COVID-19 Response Team and the Food and Drug Administration. Allergic reactions including anaphylaxis after receipt of the first dose of Pifzer-BioNTech COVID-19 vaccine – United States, December 14-23, 2020. Morbidity and Mortality Weekly Report. 6 January, 2021.