Antibody testing will be part of the next wave of information and defense against the novel coronavirus. And confirming what some have been suggesting for several weeks, preliminary results of a large-scale antibody testing effort by Stanford researchers show that the vast majority of COVID-19 cases in Santa Clara County have likely gone uncounted — and between 2.5 and 4.2 percent of county residents have already had the virus.

The raw results of the 3300-person test effort, which still need to be peer-reviewed, were reported by ABC News on Friday, and they represent the first large-scale look at the prevalence of COVID-19 antibodies in a community.

Leading the study is Dr. Eran Bendavid, an associate professor of medicine at Stanford University, and study participants were found through online ads.* Bendavid tells ABC News, "We have good confidence that we’re getting reliable information on the population. And that can be done because we know what proportion of the people who are positive we’re missing using this test."

The raw, unadjusted results of the testing found antibody prevalence of 1.5 percent, and Bendavid's team has adjusted that up based on various factors to a range of 2.5 to 4.2 percent. At the time of the tests, as SFGate notes, the county had just under 1,000 confirmed COVID cases — it now has nearly 2,000. This means, with a county population of 1.9 million, the underreporting of cases may be by a factor of 50 to 85 — suggesting that between 48,000 and 81,000 people have had the virus without knowing it, or without reporting their symptoms to authorities.

This study may further complicate the picture that is emerging about how many people catch the coronavirus and remain relatively asymptomatic — two weeks ago, the head of the CDC was putting his upper estimate of asymptomatic cases around 25 percent.

And this data also may run counter to the belief by some in the scientific community who believe the virus first arrived in the U.S. in mid-January — though it may merely mean that Santa Clara County had more cases than it realized before the state began widely testing.

Dr. John Brownstein, an epidemiologist at Boston Children's Hospital and a contributor to ABC News, cautions that this data could be skewed both by the geographical area it covered, and by the fact that researchers used online ads to find participants. But he admits, "There has been wide recognition that we were undercounting infections because of lack of testing or patients were asymptomatic."

Bendavid is quick to point out that the study, if confirmed, still suggests that 95 percent of the population has not been exposed to the virus, and this could spell trouble ahead if social-distancing measures are lifted too soon.

"Herd immunity," a term that's been tossed around a lot recently, typically means a very high percentage of the population already has antibodies or has been vaccinated. For this virus, experts estimate that herd immunity will require 80 percent of the population to be immune, and that likely can not happen without a vaccine or mass, unfathomable casualties.

As this study's results are being confirmed and reviewed, a similar antibody prevalence testing effort is going on in Los Angeles led by Dr. Neeraj Sood at the University of Southern California.

Related: Some Scientists Suggest Undetected Coronavirus Caused California's Oddly Early 'Flu Season'

Photo of Stanford by Stan Wang

*An earlier piece on SFist and ones in several other local news sources mistakenly connected this study with comments made by another Stanford figure, Hoover Institute fellow Victor Davis Hanson.