This LA Times piece is persuasive in amassing evidence that infections were popping up here and there in Cali by mid-February. But this hypothesis is more controversial:

But Smith on Friday said data collected by the federal Centers for Disease Control and Prevention, local health departments and others suggest it was “a lot longer than we first believed” — most likely since “back in December.”

“This wasn’t recognized because we were having a severe flu season,” Smith said in an interview. “Symptoms are very much like the flu. If you got a mild case of COVID, you didn’t really notice. You didn’t even go to the doctor. The doctor maybe didn’t even do it because they presumed it was the flu.”

There’s a theory kicking around online that California’s surprisingly low number of infections and deaths is due to some “hidden epidemic” of coronavirus that struck late last year, disguised as a bad flu season, and that the state may already be racing towards herd immunity now. Theories like that catch fire mainly because of wishcasting: We all want to believe that the Oxford model is right, that the disease has spread much further than we think and that it’s largely harmless to everyone except an unlucky few. The epidemic will be over much sooner than expected. But the evidence is thin. For starters, if California had a wave of COVID-19 cases this past winter, show me a gruesome death spike there that parallels what New York City is experiencing right now. There’s no reason to think the disease would be much more lethal to New York’s population than to California.

Likewise, it’s hard to square the “California is immune!” theories with the nearly universal belief that China is concealing new outbreaks in Wuhan and beyond. If Cali is immune, China must have been immune even earlier. If you think the epidemic will soon end here, you must believe that it’ll end even sooner in China — or has already ended.

Trevor Bedford is the researcher in Seattle who used genome analysis to sniff out the first evidence of “community spread” in Washington state. He’s heard the theories too that California may have had cases of COVID-19 last year. It’s nonsense, he explains in this thread. Given how much traffic there is between California and Washington state, cases cropping up in California last fall/winter should have produced some cases in Washington too. An analysis of samples taken in Washington from October 2019 through February 2020 found zero coronavirus infections.

Additionally, we confirmed that these samples from acute respiratory infections from Oct 2019 through Feb 2020 contained a variety of different viruses including influenza, RSV, rhinovirus, metapneumovirus and seasonal coronavirus. 6/18 pic.twitter.com/rjPEWFROjg

— Trevor Bedford (@trvrb) April 12, 2020

It's empirically true that the Seattle flu season in Oct through Jan was not the result of COVID-19. We would have seen it in these data. Given travel connections between WA and CA there is no way that COVID-19 was widely circulating in CA, but we see zero cases in WA. 8/18

— Trevor Bedford (@trvrb) April 12, 2020

Whether there were a few fluke cases in California in December due to travel from China is hard to say. But it’s easy to say that there was no mass infection and no herd immunity. In fact, a recent German study estimates that only a small percentage of the U.S. population has been infected so far, nowhere near the 50-66 percent needed for herd immunity.

According to a recent analysis by two German researchers, the official numbers published by 40 national governments represent just 6 percent of infections on average, meaning that “the true number of infected people worldwide may already have reached several tens of millions,” as opposed to the current global tally of fewer than 2 million…

As of March 31, Vollmer and Bommer calculate, confirmed cases represented just 3.5 percent of infections in Italy, 2.6 percent in France, 1.7 percent in Spain, 1.6 percent in the United States, and 1.2 percent in the U.K…

Vollmer and Bommer’s estimated COVID-19 IFR for the United States is close to 1 percent, which is the high end of the range that federal public health officials consider reasonable and would make COVID-19 about 10 times as deadly as the flu.

There were 551,826 confirmed cases in the U.S. as of yesterday. If that represents 1.6 percent of actual infections, that means there are upwards of 35 million Americans infected — a little more than 10 percent of the population. With more than 21,000 deaths so far, that would mean we’d have 100,000 or so more deaths to go before approaching herd immunity, right in the range that the White House model predicts. And the German study is actually optimistic in terms of how far the disease might have spread. Scott Gottlieb’s best guess based on the available information is that two to five percent of the public has had the disease, with rates of 10 percent possible among health-care workers who are frequently interacting with sick people.

Even if more are infected than we think, that doesn’t necessarily mean we’re closer to herd immunity. ABC notes that the share of the population that needs to be infected to produce herd immunity rises for diseases that are super-infectious. (For the measles, it’s 93-95 percent.) If you think COVID-19 is so easily transmitted that, say, 30 percent of Americans have been infected in two months, then we may need much more than 50-66 percent to contract it before the infection rate starts to decline. And then there’s the anecdotal evidence about some recovering patients in Asia possibly having been “reinfected.” If some percentage of people who’ve had the disease aren’t durably immune then they don’t count towards herd immunity. If that’s the case here, we’d be further away from herd immunity than we hoped even if blood tests confirm the estimate of 35 million infected.

Anyway, there’s been no herd immunity in California, safe to say. The evidence of it would be more apparently, starting with a body count.