Two studies involving antibody blood tests in California -- one in Los Angeles County and the other in Santa Clara County -- so far have suggested that the number of people already infected is dozens of times higher than officially reported.
Both studies are ongoing and have yet to be peer reviewed. Preliminary results were announced in the last few days.
Still, the studies estimate that only small fractions of the county populations have antibodies. The Los Angeles County study estimated between 2.8% and 5.6% of the population — 221,000 to 442,000 people -- had Covid-19 antibodies, based on drive-through testing conducted on April 10 and 11. That would have been 28 to 55 times the number of cases that county officials recorded around that time. In Santa Clara County, the study estimated 2.49% to 4.16% of people there had been infected with Covid-19 by April 1. That represents between 48,000 and 81,000 people -- and 50 to 85 times the cases that county officials recorded by that date.
Clearly we should not expect PCR tests to capture everyone because asymptomatic people aren't usually (randomly) tested except in small-enough communities, e.g. ships like the Diamond Princess cruise or the aircraft carrier Theodore Roosevelt. In both of these studies/cases however the asymptomatic ratio was about 50-60% at the peak of the infection... so still that leaves a giant gap to the recently reported antibodies tests from California, which reported orders of magnitude more people supposedly had the virus (and presumably had few or no symptoms).
On the other hand, Covid-19 antibodies tests are themselves been questioned for both relevance (in general) and accuracy (at least in the US, but probably elsewhere).
So, have such more widespread Covid-19 antibodies tests been already conducted outside the USA, and if so have any result (even preliminary) been reported, in particular relative to official/PCR diagnoses? (CNN also mentions ongoing antibodies studies in other US states, but says nothing outside of the US.)