By doing serologic studies you can estimate the global number of contaminated people: looking at the number of viral mutations, as well as different strands of the virus that can be identified as being generated in a specific region.

It’s going to be very imprecise. For H1N1 the global infection rate could have varied from 11% to 21% of the global population, which is a +/- 350 million people estimation.

https://www.cidrap.umn.edu/news-perspective/2011/08/study-pu...

The IFR rate for covid-19 seems to be trending towards 0.1-0.5% based on the serological tests over entire population cohorts that are starting to be published.