  I use a very simple deterministic model for the spread of Covid-19 in a large
population. Using this to compare the relative decay of the number of deaths
per day between different regions in Italy, Spain and England, each applying in
principle the same social distancing procedures across the whole country, I
obtain an estimate of the total fraction of the population which had already
become infected by April 10th. In the most heavily affected regions, Lombardy,
Madrid and London, this fraction is higher than expected, i.e. $\approx 0.3$.
This result can then be converted to a determination of the infection fatality
rate $ifr$, which appears to be $ifr \approx 0.0025-0.005$, and even smaller in
London, somewhat lower than usually assumed. Alternatively, the result can also
be interpreted as an effectively larger fraction of the population than simple
counting would suggest if there is a variation in susceptibility to infection
with a variance of up to a value of about 2. The implications are very similar
for either interpretation or for a combination of effects.
