Anaphylaxis after the COVID-19 Vaccine is Somewhere Between Extremely Rare and Non-Existent.

There is one good thing about the competitive medical research environment going on in Boston. They observe everything. Every once in a while, their observations are actually useful to us lowly non-academics.

A letter published in JAMA this week from Massachusetts General Hospital addresses the question of risk of anaphylaxis after receiving the COVID vaccine. Prior to initiating a vaccine campaign they invested in infrastructure to monitor adverse effects from the vaccine.

The goal was to monitor all of their employees for 3 days after the vaccine using symptom surveys. Email campaigns, text messages, phone calls, and a smart phone application were used to keep tabs on everyone. They were quite successful since their survey response was 81%. This is very high for any survey. Generally, you’re lucky to get around 20% of eligible participants to respond.

The department of allergy/immunology was responsible for reviewing any potential incidents of anaphylaxis. They did this by reviewing survey data and electronic health records. The likelihood of anaphylaxis was scored based on the Brighton Criteria for anaphylaxis and the National Institute of Allergy and Infectious Diseases criteria for anaphylaxis. Both are previously validated diagnostic scores used for research purposes to determine the incidence of anaphylaxis after an exposure to a potential allergen.

Here are their numbers.

There were able to collect data on 65,000 vaccine recipients. The two vaccines used at the facility were the Pfizer and Moderna vaccine.

26000, received the Pfizer vaccine. 39,000 received the Moderna vaccine.

1365 reported an allergic reaction.

Of those 1365 reactions, 16 met at least 1 criterion for anaphylaxis. The Brighton Criteria classified 14 as anaphylactic reactions. The NIAID classified 9 as anaphylaxis. They were equally distributed between the Pfizer and Moderna vaccines.

One! That’s right, One! Of these reactions met Brighton Criteria for high diagnostic certainty. The remainder were low levels of certainty. There were no grade 3 or 4 (most severe) allergic reactions based on the NIAID classification.


Mean time to onset of symptoms was 17 minutes. Yes, that’s minutes, not days.

Only 9 required an injection of Epinephrine. One patient required care in a hospital setting. That’s (1/ 65,000)

15 of the 16 reactions occurred in females.

The reported anaphylaxis rate was 2.47 per 10,000 vaccinations. Excluding minor cases, the anaphylaxis rate was 0.15 per 10,000 vaccinations. That’s a really low number.

This makes sense given the ingredients in the vaccine. There are hardly any. The Pfizer vaccine has five ingredients. The Moderna vaccine has seven. None of them are known inducers of allergic reactions. Most of the ingredients are biochemical compounds your body uses every day to build and repair cells. These are things like mRNA and lipid particles which your cells are made from.

One of the reported cases appears legit. The certainty of the other 15 cases was in question. Anaphylaxis is easy to diagnosis in severe cases. The typical presentation is a rash (commonly known as hives), tongue and lip swelling, with high heart rate and low blood pressure. Mild cases may have just one of the above. These signs occur in many other diseases. Seven of the cases improved without epinephrine which would be highly unlikely in severe cases. Of all of the cases only one reached Brighton level 1, which is the highest level of diagnostic certainty. None of the cases met NIAID criteria for severe reaction, which would be similar to the case described above.

Of the reported reactions 15/16 occurred in women. I’m not going to try to offer an explanation for this other than to say it is odd. I don't understand how a pair of ovaries and some estrogen would increase the risk of anaphylaxis. Someday, someone, much smarter than me may be able to explain this phenomenon.

When comparing the occurrence of serious allergic reactions to the COVID vaccine to the 500,000 plus deaths, the payoff is clearly asymmetric to the upside in favor of the vaccine. It's time to end this debate.

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