What does the Pfizer press release really mean?

November 9, 2020

The press release

A press release came out today from Pfizer and BioNTech SE with a announcement, you can read the full briefing here.

Suffice it to say leading news outlets all over the world have jumped on the announcement with headlines like “Milestone vaccine offers 90% protection” (www.bbc.com, 9th Nov 2020, 17:00 GMT) or “Covid-19 vaccine candidate is 90% effective, company says” (cnn.com, 9th Nov 2020, 16:00 GMT). But what’s really in the release? Let's have a look!

What does it say?

38,955 people were given two shots. Out of these, half were twice given a real vaccine in development, and half were twice given a placebo. Among all 38,955, 94 people later were proven to have contracted COVID-19: 2,4%. From the cited 90% efficacy, it can be concluded that means: Nine out of approximately 19,478 people who were given the real vaccine contracted COVID-19, while 85 from the placebo-control group did. This may mean the vaccine is effective, leading Pfizer to do this press release. Generally, this is good news. However, one has to consider that the cohort of ill people in total, 94, is still rather small. The press release also explains that the study will finish when 164 patients have contracted COVID-19. This number was likely chosen as they can be pretty certain that a low case number in the vaccine group vs the placebo group was significant at that number of cases and not merely by chance. However, we have a way to go yet: even if the vaccine does prove effective and free of bad side effects, it’s going to be a long while before enough doses can be produced to immunise the whole world. If you want to know about everything that can go wrong in detail, I would like to point you at the legal disclaimer under the press release, which is at least as long as the press release itself and contains beautiful phrases like “risks and uncertainties include (…) the uncertainties inherent in research and development” and “whether and when any such applications may be approved by regulatory authorities, which will depend on myriad factors”.

I am confused

It has to be said that even I, as a scientist, struggled to understand what this press release is saying. Many parts feel confusing, leaving deductions to the reader. For example, 43,538 people were given a first dose, but only 38,955 got a second dose. Were the 4,583 people who only got one dose included in the final results? Apparently not, as they conclude „a vaccine efficacy rate above 90%, at 7 days after the second dose.“

What are “94 evaluable cases” – what constitutes a non-evaluable case?

This becomes only clearer when looking to the clinical trial protocol which is linked from the press release. Here, the evaluable is defined as “no other important protocol deviations as determined by the clinician” and “participants complying with the key protocol criteria” - whatever that means.

What does “approximately 42% of global participants and 30% of U.S. participants have racially and ethnically diverse backgrounds” mean? I am wondering the other 58% / 70% that were not diverse were; I personally suspect that by diverse they meant non-white / non-Caucasian?


The media hype could be a bit more restrained: After all, this is science - but, all in all: Good news with many things that could still go wrong.

I would very much like to thank Sam Horrell, Dale Tronrud and Florian Platzmann for discussions and clarifications.


Andrea Thorn

Group Leader @ Institute for Nanostructure and Solid-State Physics, Hamburg University
Andrea is a specialist for crystallography and Cryo-EM structure solution, having contributed to programs like SHELX, ANODE and (a little bit) to PHASER in the past. Her group develops the diffraction diagnostics tool AUSPEX, a neural network for secondary structure annotation of Cryo-EM maps (HARUSPEX) and enables other scientists to solve problem structures. Andrea is […]
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