@Isitisntit
I’ve searched and can’t find anything from Chris Whitty on this, and can’t see any obvious flaws in my logic.
In an unsupressed environment, the total number of people infected will depend on the R number. 60% assumes an R of 2.5 which is roughly what Covid’s R is believed to be.
Of course, not everyone who is infected will have symptoms. Many, possibly most, wont, but my admittedly crude calculations aren’t based symptomatic cases, just infections.
I was careful to use an IFR or infection fatality rate figure - that’s all infections, not just those who are “sick”. The CDC’s current best estimate for the IFR is 0.65%, slightly above the figure I used! That’s broadly consistent with the U.K. estimates for infections and deaths.
Apply that to the world population, and you get the 25 million figure.