The theory goes like this: California has fewer COVID-19 cases than hard-hit places like New York because the coronavirus has spread throughout the state undetected since the fall and most Californians are now immune.

Turns out it’s too early to prove the unlikely notion started by a Stanford University military historian who published an article about it in a national magazine that conservative talk show hosts and California media outlets quickly picked up. In fact, public health experts say there’s a far more probable explanation for California’s comparatively smaller case load of 20,491 compared with New York’s 170,512: The state’s early shelter-in-place orders have so far prevented many Californians from being exposed to the coronavirus. But without social isolation, they say, many Californians could still get sick.

“We’ll still have a largely vulnerable population because people are not immune,” said UCSF’s Dr. Charles Chiu, one of the top infectious disease experts in the world. “The only way we’re able to control the virus is with a vaccine.”

The idea that many Californians are immune to COVID-19 started with Victor Davis Hanson, a senior fellow in military history at Stanford’s Hoover Institution. Hanson wrote a March 31 think piece in the conservative National Review positing that the virus arrived in the U.S. from China in the fall and infected so many people that they developed immunity to the disease that would come to be called COVID-19. This massive level of protection from reinfection — a so-called “herd immunity” — could be responsible for California’s relatively low case rate, Hanson hypothesized. He concluded that “we won’t know the answers” until researchers conduct widespread testing to find out who had the disease.

Conservative talk show hosts Laura Ingraham and Rush Limbaugh jumped on the theory.

“I think they’ve immunized themselves,” Limbaugh said about Californians on his nationally syndicated radio show a week ago - and credited Hanson with the idea.

This week, Monterey TV station KSBW, which is owned by the Hearst Corp., which also owns The Chronicle, interviewed Hanson. That’s when the theory about “herd immunity” spread locally to Bay Area stations and online news sources — including SFGate, also owned by Hearst, which published KSBW’s story (SFGate’s editorial director, Grant Marek, removed the article after questions about its accuracy came to light.)

The Chronicle contacted Hanson to ask what evidence backed up his theory. Through a Stanford University spokeswoman, Hanson pointed to a Stanford Medicine study that collected blood samples from Santa Clara County residents to determine whether they have antibodies to COVID-19. Antibodies appear when a body has fought a virus.

But the study’s results aren’t available yet, a Stanford Medicine spokeswoman said. She could not comment on whether the researchers were investigating herd immunity.

It is also not known yet whether people who have recovered from COVID-19 are immune from getting it again, although antibody tests will offer clues. Most — but not all — viruses do confer immunity once the patient recovers, so medical researchers hope people will be immune to a recurrence of COVID-19. But more research is needed.

Stanford Medicine’s Dr. James Zehnder, who helped to develop antibody tests being given to health care workers, said that there is not enough information to come to any conclusions on whether many Californians are already immune to the disease. He is not connected to the study that Hanson cited.

The coronavirus that causes COVID-19 was first identified in Wuhan, China, in December, where scientists believe it jumped from bats to humans. The severe respiratory illness has since spread around the world.

UCSF’s Chiu, a leading scientist tracing the disease, published research with the Centers for Disease Control and Prevention and the California Department of Public Health showing that numerous people brought the coronavirus into Northern California throughout January and February. The studies offered no evidence that the coronavirus arrived in the fall, as Hanson suggested.

Chiu conducts his research by tracing different strains of the disease. He connected the lineage of cases from the Grand Princess cruise ship that docked in Oakland in March to the first reported U.S. outbreak in Washington state in January. He discovered that the case of a traveler returning to California from New York was linked to strains circulating in Europe. And he revealed that an outbreak cluster in Santa Clara County had come from a strain from China.

None of the medical researchers who spoke to The Chronicle said evidence exists to support the idea that the virus was infecting Californians in the fall.

“That’s overstated. We would be seeing thousands of thousands and thousands of cases if this has been circulating longer,” said Dr. George Rutherford, another UCSF infectious disease specialist.

In order for large-scale immunity to protect people who are susceptible, at least 40% of a population needs to be immune, multiple infectious disease experts said. For the measles, that proportion is as high as 95%, Rutherford said. He added that it takes years to build up mass immunity for the flu.

So far, COVID-19 is known to have infected less than 1% of California’s population. But California still has one of the lowest rates of testing per capita in the country, so the true rate may be higher.

UCSF is also testing blood from non-COVID patients who got routine lab tests in San Francisco to get a sampling of how many people might have recovered from COVID-19 without knowing they had it. Dr. Bryan Greenhouse, a UCSF assistant professor and medical researcher with the Chan Zuckerberg Biohub in San Francisco, said that even if 10% of those people had been infected with the coronavirus, it wouldn’t prove whether massive immunity had taken hold in the region.

Greenhouse said his educated guess was that the Bay Area has kept cases relatively low because of the early protections imposed by public health officials — and not because most people are immune.

“But we’re certainly open to finding out,” Greenhouse said. “The only way is to measure it.”

Mallory Moench is a San Francisco Chronicle staff writer. Email: mallory.moench@sfchronicle.com Twitter: @mallorymoench