Interim CDC analysis of severe allergic reactions in people who received the Moderna 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):

“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:


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


  • “nausea, vomiting, diarrhea, abdominal pain”


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


  • “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 Moderna vaccine

The interim CDC analysis is based on the rollout of the first 4,041,396 doses of the vaccine. After assessing reports of adverse events from this data set, the CDC has concluded that there were 10 cases of anaphylaxis. This results in a rate of about 3 cases per one million doses. This is very rare.


A brief profile of the 10 people who had severe allergic reactions is as follows:

  • age range – 31 to 63 years
  • all were women (this point is revisited later)
  • time to allergic symptoms – some people developed allergic symptoms as quickly as one minute after their injection, while others took as long as 45 minutes.

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

  • within 1 to 15 minutes – 90%
  • after 30 minutes – 10%

All affected patients received intramuscular injections of epinephrine (adrenaline) after their symptoms appeared.

Six patients were hospitalized, including five who were admitted to an intensive care unit. Four patients who were in the ICU had to have a flexible plastic tube inserted into their throat so they could breathe.

Complete data were available on nine of the 10 patients.  All nine people recovered and were sent home. No one died.

A history of allergic reactions

The CDC researchers found that nine of the 10 patients had a history of allergies or allergic reactions, including to drugs or medical products (such as contrast media used during high-resolution scans) and foods. Five patients had previously experienced anaphylaxis in the past, but none of these episodes was in reaction to a 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 43 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 sensations in the throat
  • sensation of throat closure
  • mild respiratory symptoms (the agency did not define these)

Patients with these allergic symptoms were between 22 to 96 years old. About 91% 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 24 hours. The distribution of the onset of these symptoms after vaccination was as follows:

  • within 30 minutes – 75%
  • after 30 minutes – 25%

According to the CDC, in 60% 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 Moderna vaccine, 61% of the people vaccinated 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


CDC COVID-19 Response Team and the Food and Drug Administration. Allergic reactions including anaphylaxis after receipt of the first dose of Moderna COVID-19 vaccine – United States, December 21, 2020 to January 10, 2021. Morbidity and Mortality Weekly Report. 22 January, 2021.