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 has already become infected  . 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  .
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 .
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. 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