The number of people that are infected doesn't tell you that much about R0. You can get R0 from the increase in daily new deaths and the generation time between

The number of people that are infected doesn't tell you that much about R0. You can get R0 from the increase in daily new deaths and the generation time between infections, given that the quality of care stays constant. Serological tests will tell you something about the early outbreak and how many cases where not detected and how high the infected fatality ratio really is. I have seen many people that still believe that the IFR is in the range of an average flu. With the data from South Korea, we can put a number at what we can expect the IFR to be if we make some optimistic assumptions. That is why I say that you should not expect that serological tests show that a large part of the population already had covid19 and that the IFR is much lower than we expect. They will show where exactly the IFR is.