One of the great unknowns of the Covid-19 crisis is just how deadly the disease is. Much of the panic dates from the moment, in early March, when the World Health Organisation (WHO) published a mortality rate of 3.2 per cent – which turned out to be a crude ‘case fatality rate’ dividing the number of deaths by the number of recorded cases, ignoring the large number of cases which are asymptomatic or otherwise go unrecorded. 

The Imperial College modelling, which has been so influential on the government, assumed an infection fatality rate (IFR) of 0.9 per cent. This was used to compute the infamous prediction that 250,000 Britons would die unless the government abandoned its mitigation strategy and adopted instead a policy of suppressing the virus through lockdown. Imperial later revised its estimate of the IFR down to 0.66 per cent – although the 16 March paper which predicted 250,000 deaths was not updated.

In the past few weeks, a slew of serological studies estimating the prevalence of infection in the general population has become available. This has allowed professor John Ioannidis of Stanford university to work out the IFR in 12 different locations.