By now you've likely had a friend on social media talking about how they think they have already had COVID-19, and it happened a month or two before it was declared a pandemic or even known to have reached the U.S. Some Stanford researchers are now taking a broad-based look at the possibility that the coronavirus arrived here — in California at least — much sooner than the current wisdom suggests, leading to some herd immunity that has kept our state's case numbers low.

A man in Washington State has been called Patient Zero for the U.S. outbreak, and he arrived home in the Seattle area in mid-January after traveling to Wuhan, China. That case was reported on January 21 when China was reporting only 300 cases of the virus and six deaths. The virus samples taken at a nursing home in the Seattle area six weeks later were traced back to this Patient Zero, and genetic sequencing has traced the Grand Princess cruise ship cases back to the Washington state outbreak as well.

California's first detected and confirmed cases of COVID-19 were in Orange and Los Angeles counties in late January, and the Bay Area's first case, in Santa Clara County, was reported on January 31. San Francisco confirmed its first two cases almost six weeks later, on March 5.

Meanwhile, the first cases were being detected in New York State, in particular with an outbreak in a Jewish community in New Rochelle.

But in the barely five weeks since, New York State has experienced 14 times the number of deaths as California has, despite having half the population of California. Some of this might be explained by the density of New York City and the delay in locking the city down after the cluster in New Rochelle, which is just outside Manhattan. But epidemiologists believe something else must be at play to explain California's relatively lower hospitalization and death rates — taking it as a given that there have likely been tens of thousands more cases of the virus here that have gone undetected because they were asymptomatic or mild.

People obeying social distancing has helped, but the Stanford researchers are taking the hypothesis that the Chinese outbreak started earlier than China has said, and that cases of the novel coronavirus began popping up in California in December or earlier, and were just diagnosed as influenza.

As KSBW reported this week, Victor Davis Hanson, a senior fellow with the Hoover Institution at Stanford, has been discussing whether herd immunity to the virus existing in California before what we think of as the current outbreak began. And other researchers at the Stanford School of Medicine are separately doing antibody tests on random people, starting with 3,200 volunteers at three testing sites in the Bay Area last weekend.*

"When you look at other states, it doesn't quite explain completely why California has been more fortunate, especially when it should be the least fortunate," Hanson says, speaking to ABC 7. He adds, "Something is going on that we haven't quite found out yet. When you calculate as well there were people on direct flights [in December and January], from San Francisco and LAX to Wuhan, ground zero of the outbreak, you'd be naive not to think the California population wasn't exposed."

Update: As the LA Times reports, antibody testing being funded by the National Institutes of Health is "looking for virus antibodies in samples from blood banks in Los Angeles, San Francisco and four other cities across the country." And more experts are coming around to the idea that community spread of the virus in the Bay Area as ongoing long before the first COVID-19 tests were administered.

Last weekend's antibody testing is being followed up with 1,000 more in Los Angeles County, happening at six testing sites, conducted by USC and the Los Angeles County Department of Public Health.

People develop antibodies with two to 15 days of showing symptoms from the coronavirus, according to UCSF researchers who are also conducting their own antibody testing. As ABC 7 reports, the UCSF team is hoping to test healthcare workers across San Francisco to determine if they are likely now immune to the virus and therefore more able to confidently work on the frontline treating patients.

Hanson's hypothesis is contradicted in a New York Times piece published Thursday, concerning the ongoing genetic sequencing being done on virus samples. Researchers have determined that the New York outbreak came from Europe, and based on the rate at which the virus is mutating, they have concluded that the U.S. outbreak must have only began in January, as has already been reported.

Per the Times:

Tracking viral mutations demands sequencing all the genetic material in a virus — its genome. Once researchers have gathered the genomes from a number of virus samples, they can compare their mutations.

Sophisticated computer programs can then figure out how all of those mutations arose as viruses descended from a common ancestor. If they get enough data, they can make rough estimates about how long ago those ancestors lived. That’s because mutations arise at a roughly regular pace, like a molecular clock.

Based on that clock, Dr. Trevor Bedford, an associate professor at the Fred Hutchinson Cancer Research Center and the University of Washington, says that he is "quite confident that it was not spreading in December in the United States." Regarding that Patient Zero in Washington, which his team used to compare genetic data with later cases nearby, he concedes that the case may not have been the only Patient Zero in the U.S. at the time. "There may have been a couple other introductions [of the virus] in January that didn’t take off in the same way," Bedford tells the Times.

Herd immunity saves some lives but it does not represent a reason to be complacent. A large number of undetected, early cases in California would only have meant that more people have been sick over a longer span of time, contributing to a curve-flattening that was occurring sooner than on the East Coast.

And, again, some of the severity of the New York outbreak has to be attributable to the density of urban conditions there. Viruses love human density, and California doesn't have anything resembling New York City in terms of an urban population, even if the state's population is double that of New York's.

Related: Santa Clara County Resident Is First Bay Area Case of Coronavirus

Grand Princess Coronavirus Cases Likely Linked To Seattle Cases

Photo: Ibrahim Boran

*This post has been corrected to show that Hanson's team is not leading the antibody prevalence testing; that is being done by the Stanford School of Medicine.