There is certainly a significant amount of underreporting of coronavirus cases. This is due to people with severe symptoms being more likely to be tested, and the presence of a substantial asymptomatic fraction (around 50% according to some estimates, but likely lower after accounting for false positive rate). Underreporting should show up as the official CFR (case fatality rate) higher than the true CFR (if everyone was tested). The rationale is that if only (or disproportionately) very sick people are tested, the CFR will be higher because very sick people are more likely to die than people with mild or no symptoms. Another indicator of underreporting is the positivity rate (% tests that come up positive). If all individuals of two populations are tested, the positive rates are true reflections of the % infected individuals. However, since authorities only test a sample of the population, the positive rate depends on the criteria that people must meet in order to be tested (i.e. number and type of symptoms, contacts with confirmed cases, etc.). Hence, if both CFR and positive rate are affected by testing criteria (how much testing is restricted only to people with severe symptoms), there should be a positive correlation between them. A positive correlation would provide evidence for high CFR being an indicator of underreporting.
So far, 361,060 tests have been carried out in Italy. This is not a small number compared to other countries, but not large enough considering the amoung of infected individuals. Large scale testing is badly needed, considering the high positivity rate.
Covid-19 test statistics (http://www.salute.gov.it/nuovocoronavirus).
The CFR goes from 2.8% in Sicily to over 14% in Lombardia. This is a big difference. There is a lot of variability in the positivity rate as well, ranging from 6% in Calabria to 40% in Lombardia.
There is a positive correlation between CFR and positivity rate across 21 regions (r= 0.76).
However, this correlation supports the hypothesis that there is severe underreporting of cases.
The CFR in Italy is 10% and if we assume a true CFR around 1%(data from Diamond princess), then in Italy there are 10 times more cases than the confirmed ones.
Age could be a factor mediating the difference in CFR. In Italy covid-19 infects the elderly more than young people, as shown by a population-wide study of Vo’s (95% response rate). I don’t have data broken down by age for the Diamond Princess, but a correction of the CFR should be done according to it. Even after accounting for different age profile of infected individuals in Italy and on the Diamond Princess, there is likely a CFR difference between the two populations. I believe this difference is a result of under-reporting.
At the world-wide level, the correlation between CFR is r= 0.47, but the two variables are skewed, due to the highly heterogeneous nature of the samples.
Piffer 