FDA Pfizer/BioNTech vaccine efficacy results are great, but aren't nearly as great as presented for severe infections.
Everyone has seen fig below on cases in vaccine (blue) & placebo (red) over time.
Thread.

https://www.fda.gov/media/144245/download

Key aspect of graph: x-axis. It shows days since dose 1 was given. As expected, no difference in total symptomatic cases in vaccine vs placebo for first 7-10d. It takes some time for vaccine to have any effect!

In fact, main reported efficacy of ~95% is for cases starting 7d after 2nd dose. Efficacy for earlier time points are lower: after dose 1, efficacy is 82%.

Why does this matter?

Because the 2 most important effects of vaccine will be to reduce severe infections & block transmission. What data do we have on these two endpoints that is comparable to the 95% efficacy? A: Nearly none and none at all.

But wait - didn't they report difference in severe cases b/w vaccinated & placebo in press release? Yes, they did, but data aren't so simple. If we use same period to assess efficacy at preventing severe cases (7d post 2nd dose), data evaporate - most sev cases before this date.

Look at data carefully. Only 3 severe cases >7d after 2nd dose in placebo (dark red S dots) & 1 in vaccinated (dark blue @ day 60). This is not remotely enough data to argue vaccine protects against severe cases:
95% CI: -124%, 96.3%

So what about press release?

Press release was based on severe cases in all people, starting after dose 1 was given. This is cherry picking the data. They can't simultaneously report a 95% efficacy of vaccine overall for mild cases 7d post dose 2 & 89% for severe cases (9 of 10 were in placebo) post dose 1.

10 severe cases since 1st dose, but some of these almost certainly cannot have been prevented by the vaccine. 2 of them were on days 6&8 post 1st dose when no diff in mild cases.
(Note: I only count 8 severe cases on graph total; not sure when other 2 occurred in placebo. Help?).

Take home:
Pfizer/BioNTech vaccine definitely greatly reduces mild cases of COVID: 95% starting 7d after 2nd dose.
But does it reduce severe disease?
Not enough data to assess. Possibly. Probably? Hopefully?

Does it reduce transmission? No data at all. This text is under Data gaps. This is huge issue & fundamentally determines vaccine allocation w/ limited doses (e.g. if vacc blocks transmission, SNF staff key; if not, SNF residents).
https://twitter.com/DiseaseEcology/status/1334215150333673472

FDA doc indicates this is something that will be done post EUA. That seems crazy to me, since vaccine allocation is happening now! This data is critical for getting vaccine to right people to prevent the most deaths.