Allergic reactions to the COVID-19 vaccines

Even though I’m a vaccine developer, a nano-particle investigator, and an allergist, I have not yet commented on the reports of allergic reactions to the COVID-19 vaccines. While there are very few of these reactions, they have gained significant attention. When I saw a report on the front page of the Wall Street Journal web edition today of an allergic reaction in New York City, I felt that I needed to make some remarks.

The overall safety of both of the Pfizer and Moderna vaccines is remarkably good. Almost all the adverse events observed with either vaccine were localized to the area of the injection or involved low grade fevers and malaise that resolved spontaneously. It is interesting that in the Pfizer trials and the Moderna trials, where together approximately 40,000 people actually got two doses of these vaccines, there were no systemic reactions (anaphylaxis) or localized allergic reactions. Individuals allergic to vaccines or vaccine components were excluded from these trials, however, still it suggested allergic reactions to these vaccine were rare.

We now have reports of approximately 6 individuals who’ve had significant, systemic reactions to these vaccines. Almost all of these reports were with the Pfizer vaccine because that’s the one that has been available. Since we’ve now immunized over a million individuals in the U.S. and significant numbers outside the United States, we are looking at events that are uncommon and occurring at a frequency (less than 1 in 100,000) that makes it unlikely they would occur in the clinical trials (40,000 subjects).

What is causing the reactions? The simple answer is we don’t know. There are stabilizers in these vaccines (polyethylene glycol polymers, or PEG) that had been associated with allergic reactions against other medicines. However, the exact chemical structure of the PEG in these vaccines is different from the other medications where allergies have been documented.

There is also the possibility that the vaccine could be causing a reaction because the recipient has either a genetic tendency or a medical problem that predisposes them to having allergic reactions. These can occur without a known allergic substance. It is also possible that the vaccine wound up in the wrong place, like a vein, which allows the vaccine to contact the bloodstream. This could trigger an allergy-like reaction without an actual allergy to the vaccine. So the possibilities are many.

While we don’t know what caused these reactions, the answer is likely in the blood and urine of the individuals who had the reactions. Looking at these fluids should allow a determination of whether these were truly allergic reactions and what triggered them. It would be most helpful if these patients were studied extensively to give us some idea of what has happened.    

So, what does this mean in terms of the safety of the vaccine? The frequency of these reactions is low, and although some have been significant, all have responded to treatment. The occurrence of these reactions is also in line with those of allergic reactions to other vaccines. So, the presence of these reactions does not suggest that these vaccines are not safe or less safe than any other vaccine.

One thing that’s clear is that there is no protein in this vaccine that would cause a reaction in someone that was allergic to food or pollen. That is why when the British initially suggested that severe food or drug allergies were exclusion criteria for the vaccine, the United States FDA did not concur.

In summary, the allergic reactions to the approved COVID-19 vaccines are uncommon and not beyond the expectations of other vaccines for infectious diseases. While we still don’t know the cause of these reactions, intensive study of the patients that had these reactions will be important to define what happened. I will likely be involved in a federal study examining allergic reactions to COVID-19 vaccines and believe that in short order we will have a better understanding of these adverse events.

In the meantime, there is no additional concern for anyone who is receiving one of these vaccines unless they have any allergy to other vaccines or a known allergy to a component of the COVID-19 vaccine.