The US is doing a considerably increased amount of testing. Also in those states, they are testing everybody who goes to hospital no matter for what reason and it's a nosocomial virus. Tests are also flawed.
https://www1.racgp.org.au/newsgp/clinical/evidence-suggests-pandemic-coronavirus-may-be-seas
https://www.reuters.com/article/us-health-coronavirus-italy-antibodies/over-half-of-people-tested-in-italys-bergamo-have-covid-19-antibodies-idUSKBN23F2JV
https://medicalxpress.com/news/2020-06-immunity-covid-higher-shown.html
https://theconversation.com/coronavirus-could-it-be-burning-out-after-20-of-a-population-is-infected-141584
https://drive.google.com/file/d/1Ws6wCd7q2yeb8qwCpQ9-3j_-5923mC8d/view
