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Comment by LatteLazy | Hacker News Reader

Pfizers stage 3 vaccine trial have 41,000 participants. I'm not sure how you got 200, have I misunderstood you?

https://www.pfizer.com/news/press-release/press-release-deta....

Yes, you have. The efficacy estimate was based on roughly ~200 confirmed cases, which your link confirms for the Pfizer vaccine.

The problem is the confirmed cases are the primary part of determining efficacy. The people who do not get the virus have a smaller impact on that statistic at higher numbers. EDIT: User runamok explains it well.

[1] https://www.cdc.gov/chickenpox/outbreaks/downloads/appx-f-in...

The relevant passage is below. Basically it's a funnel, so It takes testing a large amount of people to tease out just a handful of infections. 162 cases for placebo vs. 8 in the vaccine group.

Analysis of the data indicates a vaccine efficacy rate of 95% (p<0.0001) in participants without prior SARS-CoV-2 infection (first primary objective) and also in participants with and without prior SARS-CoV-2 infection (second primary objective), in each case measured from 7 days after the second dose. The first primary objective analysis is based on 170 cases of COVID-19, as specified in the study protocol, of which 162 cases of COVID-19 were observed in the placebo group versus 8 cases in the BNT162b2 group. Efficacy was consistent across age, gender, race and ethnicity demographics. The observed efficacy in adults over 65 years of age was over 94%.

To be clear, I am using the example of the current vaccines to prove the point that N=200 allows statistical inferences to be drawn.

The 200 you mention are comparable to the 6 in the comment above, not the 152.

That would make sense because how many of these 41000 people would have been infected without the vaccine? 200 sounds quite reasonable.

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