Allergic Reactions to Pfizer's Vaccine Were Inevitable, and it's No One's Fault

By Josh Bloom — Dec 10, 2020
After Pfizer's COVID vaccine was administered in the UK, two allergic reactions surfaced. Who’s to blame? The drug maker? I argue no. Pfizer could not possibly have known in advance whether these reactions would occur or, if so, how frequently. Why's that? Here's why.
Image: Pixy

In medicine, when something goes wrong and the finger (and deposition) pointing starts. Who's to blame?? There has to be someone.

In the case of the two serious allergic reactions to its COVID vaccine, Pfizer is the obvious candidate, but before you start blaming them consider this. There was nothing that could have been done differently. The allergic reactions were inevitable, but not preventable. Huh?

The problem is that Pfizer was caught in a circular problem. Here's why.

It's called inclusion and exclusion criteria, which is pretty much what it sounds like: who gets into a given clinical trial and who does not. This is all spelled out in great detail on a page Pfizer dedicated to COVID vaccine trials.

Although the inclusion and exclusion criteria can be different depending on the phase of the trial I'll keep this as general as possible. First, the inclusion criteria:

  • Male or female participants between the ages of 18 and 55 years, inclusive, 65 and 85 years, inclusive, or ≥12 years, inclusive, at randomization (dependent upon study phase). Note that participants <18 years of age cannot be enrolled in the EU.
  • Participants who are willing and able to comply with all scheduled visits, vaccination plan, laboratory tests, lifestyle considerations, and other study procedures.
  • Healthy participants who are determined by medical history, physical examination, and clinical judgment of the investigator to be eligible for inclusion in the study.
  • Participants who, in the judgment of the investigator, are at risk for acquiring COVID-19.
  • Capable of giving personal signed informed consent


In other words, you have to be willing, able, old enough, and healthy enough to give informed consent, in case anything goes wrong. This is rather straight-forward.

But the exclusion criteria are not so straight-forward. This list contains more than two-dozen conditions (453 words) that prevent people from participating in a trial. But we don't need to look at all 453, only 22. 

This makes perfect sense. If I'm designing a vaccine trial or a member of the institutional review board that is reviewing the trial that condition absolutely has to be one of the exclusion criteria. After all, when a company is conducting a clinical trial to determine the safety and efficacy of a new drug or vaccine how can they include people who are high-risk? They can't.

This would be like conducting safety studies for a new penicillin antibiotic without asking first whether anyone trying to enroll is allergic to penicillin. 

Is Pfizer to blame for these two bad reactions? I would argue no. The company couldn't include people who have had allergic reactions in the past because the clinical trial would then be unethical, so it could provide information about the frequency and severity of allergic reactions following vaccination. Likewise, the MHRA (the British version of the FDA) is not at fault here.

Like I said – a circular problem. But also one that raises some interesting ethical questions.


Josh Bloom

Director of Chemical and Pharmaceutical Science

Dr. Josh Bloom, the Director of Chemical and Pharmaceutical Science, comes from the world of drug discovery, where he did research for more than 20 years. He holds a Ph.D. in chemistry.

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