Like I said, I’m an optimist. So, what’s the most positive spin I can give this. It may be the complete opposite to what it appears. That is, covid19 is actually so mild that it doesn’t leave a strong enough immune signal across the whole population to be robustly measured. https://twitter.com/n_equals_42/status/1260685158325334017

For example, everyone below say 30 gets it so mild, if at all, that the adaptive immune system (the bit that generates the antibodies that are detected in these serosurveys) is barely engaged

My bad. Looks like the French study used modelling to get its IFR. The general point about the weaknesses of all these serosurveys still stands, though