Seroprevalence reports from around Europe (Sweden even...) suggest that so far no country has yet seen more than 10% of its population infected with SARS-Cov-2.
Herd immunity was always a dodgy concept. In the UK it would have meant 40m people with the virus and hence approximately 400,000 deaths.
Commentators now talk about the Imperial College model as something that has been ‘discredited’, but that is basically what it was saying and at just the time Boris was jabbering on about herd immunity.
You don’t actually need a complex algorithm to do the arithmetic.
The only way around this was to do what the Swedish ‘experts’ were doing which was to assume, for no particularly credible reason, that many more people had already become invisibly infected and that the rate of mortality was therefore a much less worrisome obstacle to the goal of herdy-gurdy immunity.
Sometimes we are actually going to have to believe the Chinese (and the WHO) when they supply the rest of the world with data, because in this case it has been spot on.