Stanford is studying whether CA has some herd immunity. https://www.kcra.com/article/new-study-investigates-californias-possible-herd-immunity-to-covid-19/32085795
My nurse sister in law shared several weeks ago now that her hospital in downtown Sac had floors on isolation (in regular protocol terms) packed with flu patients (at this point I'd say suspected flu patients).
Some key points:
Hanson said he thinks it is possible COVID-19 has been spreading among Californians since the fall when doctors reported an early flu season in the state. During that same time, California was welcoming as many as 8,000 Chinese nationals daily into our airports. Some of those visitors even arriving on direct flights from Wuhan, the epicenter of the coronavirus outbreak in China.
For years California has been the No. 1 travel destination for Chinese tourists in the United States. Even after the U.S. halted flights from China this winter Chinese travelers were still able to come to California on flights from Europe and Canada.
Hanson said through all of this the Chinese have been disingenuous about the timing of the initial outbreak of COVID-19. "They originally said it was in early January, then it got backdated to December and then early December and now they are saying as early as November 17," said Hanson.
If Californians were exposed earlier than the rest of the country to COVID-19 we may have had a chance to build up some herd immunity to the disease. We won't know if that is the case until results from the Stanford Medicine study come back.
eta: I wouldn't be surprised if, after a while, it's confirmed there's a widespread milder strain. Although it's true there's lots of elderly people in Lombardy province in Italy, if CA has had it in a diffuse manner, that province must have experienced a worse strain to have the situation they did.