[nudephil (OP)]

Here's a question that was sent in by Richard:

My understanding of the Pfizer/BioNTech efficacy claim is that it is based on eg 9/94 cases of symptomatic COVID-19 infections in the active/placebo groups. I'm struggling with this simple data handling as it should be 9:83?

A simple t-test would show a highly significant difference so that piece of news is good.

However, no data is probably ever going to emerge re the frequency of asymptomatic virus patients in the two groups, so is it acceptable to take 90% efficacy as a given?

[set fair]

The figures: I've heard that the vaccine and placebo group weren't equal, but this was suggested by working back from the figures rather from released data.

One presumes that asymptomatic cases will have a lower peak viral load. But, I understand the protection "takes 3 to 5 days" so vaccinated people may be able to pass on the virus -typically viral replication peaks about day 5. We've had some good news, now we have to wait.