Coronavirus: What's herd immunity? Stanford researchers investigate potential impact on California
New York state has half the population of California but has experienced 14 times as many deaths from novel coronavirus.
RELATED: Californians may have developed some herd immunity to coronavirus last year, Stanford team theorizes
Researchers are theorizing that herd immunity may play a role in the state's slow spread. Here's a breakdown of what this means and the pitfalls of achieving herd immunity as a country.
What is herd immunity?
Herd immunity means a fraction of the population needs to be immune to a disease to make person-to-person transmission extremely unlikely or even impossible.
In other words, as COVID-19 continues to infect humans, more will recover and become immune. The virus would therefore have more trouble spreading, and the "herd" will be more protected over time.
When will Americans develop herd immunity to the coronavirus?
The threshold for achieving herd immunity varies depending on the virus. The measles, for example, has a high threshold: up to 95%.
Yet for the novel coronavirus, that percentage appears to be much lower.
"The critical threshold for achieving that herd protection for COVID-19 is between 50% and 66%," according to Dr. Justin Lesser, an epidemiologist at Johns Hopkins University.
The bad news...
At the moment, most populations are still way below that threshold.
While the real number of those infected with COVID-19 is believed to be much higher than the number of confirmed cases, Dr. Scott Gottlieb, the former head of the U.S. Food and Drug Administration, estimates that the number of Americans who contracted the disease is between 1 and 5 percent.
"My informal survey of thought leaders in this area puts estimate anywhere from 1-5%. Ultimately we must confirm this with serology studies (which are underway). But it falls short of the 50-70% needed to achieve "herd" immunity," he tweeted.
So in order to achieve herd immunity, many people need to become infected, and this means many people would die. This can also take a huge toll on the nation's health care system.
"What really matters is how you get to herd immunity," Dr. William Hanage, an epidemiologist at Harvard University, told ABC News. "And it will be impossible to get there without a large number of deaths."
How could herd immunity be possible for California right now?
It will be impossible to know how many people developed immunity to COVID-19 before widespread antibody testing becomes available.
Yet researchers at Stanford University have theorized -- not proven -- that coronavirus first hit California undetected last year, much earlier than anyone realized, and was only seen at that time as a particularly nasty and early flu season.
"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," said Victor Davis Hanson, senior fellow with the Hoover Institution at Stanford University.
The state had a significant amount of travel to and from China last year -- some 8,000 Chinese visitors a day at California's airports. Wuhan, China is considered the origin point of the novel coronavirus. It first came to public attention there in December 2019, but researchers are looking at whether it had been around much earlier than that.
"Something is going on that we haven't quite found out yet," Hanson said. "When you calculate as well there were people on direct flights, 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."
To investigate that idea, researchers are testing people for antibodies that indicate a previously undiagnosed infection that was defeated by the body's immune system.
The Stanford researchers tested 3,200 volunteers at three testing sites in the Bay Area. They are expected to publish the conclusions of their study in several weeks.
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