New tests can identify people who have been infected and recovered from COVID-19 by searching for coronavirus antibodies in the blood.

Researchers from Stanford University recently tested 3,300 Californians in Santa Clara county for antibodies.

Their results suggest that the number of people who have been infected in the county is between 50 and 85 times higher than the confirmed case total.

Experts said that these findings could mean that the COVID-19 death rate in the US may be lower than current data suggests.

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To start opening the US back up and lifting lockdowns, experts need to have an accurate sense of how many Americans have been infected with COVID-19.

Lack of adequate testing, and the prevalence of asymptomatic carriers who are infected but never sicken, have made it hard to pin down this statistic. But antibody tests can help determine whether a person already had COVID-19, regardless of whether they ever showed symptoms.

Researchers from Stanford University tested 3,300 Californians from Santa Clara County for coronavirus antibodies to understand better how many people in the area have already been infected.

Their results, published Friday in a study that has yet to be peer-reviewed, suggest that in Santa Clara County, in early April, the number of people who are or have been infected with COVID-19 is 50 to 85 times greater than the number of confirmed cases.

The study "suggests that just looking at confirmed cases significantly underestimates the prevalence of COVID-19 in the general population even more than we thought it did," Ron Fricker, a statistician at Virginia Tech College of Science, told Business Insider.

The number of infected Americans is likely much higher than confirmed reports suggest

Tien Ngyuen wears a mask as she shops at the 168 Market in Alhambra, Calif., Friday, Jan. 31, 2020. AP Photo/Damian Dovarganes

At the time of the study, Santa Clara County had the largest number of confirmed cases of any county in northern California: 1,094.

Between April 3-4, the study authors tested 3,300 county residents who were recruited using Facebook ads that targeted participants from various zip codes and demographics.

"It is a very intriguing and provocative study done by good people who have made a reasonable effort to get an appropriate community sample," William Schaffner, a professor of preventive medicine and infectious disease at Vanderbilt University, told Business Insider.

The researchers tested both adults and children, finding that 1.5% of the people tested had antibodies. They then used that result to extrapolate their findings to a population-level and found that between 2.5% and 4.2% of Santa Clara County residents would have coronavirus antibodies.

That suggests the number of people who have been infected there since the outbreak began is between 48,000 and 81,000 — a figure that's between 50 and 85 times more than the number of confirmed county cases.

Both Fricker and Schaffner agreed that the researchers' statistical analysis is accurate and appropriately done. However, Fricker pointed out that the results are specific to one geographic location in the US at one point in time.

Schaffner did suggest that the data could "indeed be roughly predictive of most of the country."

More coronavirus cases means a potentially lower death rate

"If there is good news in these results, it is that the higher prevalence also suggests COVID-19 may have a lower infection fatality rate," Fricker said.

Death rates, or infection fatality rates, from the coronavirus — a calculation that divides the number of known deaths by the confirmed case total — vary widely by country and fluctuate over time. Some health experts have predicted that death rates will decrease as the number of cases rises and testing continues to expand.

That seems to be the case in Santa Clara County. On Tuesday, the US's coronavirus death rate was 4.1%, but the Stanford researchers wrote that their findings correspond to an infection fatality rate of 0.12% to 0.2%. According to Fricker, that's similar to the fatality rate for a bad flu season in the US.

"Of course, it's important to keep in mind that COVID-19 fatality rates also critically depend on whether the local medical system is able to adequately handle the number of cases," he added.

The death rate of a disease is different from its mortality rate — the latter is the number of deaths out of the number of people in an at-risk population. A death rate is not a reflection of the likelihood that a given person will die.

The value of looking for antibodies

The tests that the Stanford scientists used differ from the diagnostic tests used to determine whether someone has an active COVID-19 infection. The latter involves taking samples of mucus and saliva and running a test in a lab to see whether those samples contain the coronavirus' genomic sequence. The results can take one to two days.

Antibody tests, by contrast, can tell within minutes whether a person has coronavirus antibodies — similar to the way home pregnancy tests and HIV antibody tests work. A kit includes a needle (to prick your finger), a 3-inch mixing stick, and a test solution. A positive result would mean someone is probably immune to reinfection for at least some time.

Read more: Tests that can tell if you're immune to the coronavirus are on the way. Here are the companies racing to bring them to the US healthcare system.

It's like dusting for its fingerprints after a crime, rather than catching the virus in the act.

A scientist presents an antibody test for coronavirus in a laboratory of the Leibniz Institute of Photonic Technology (Leibniz IPHT) at the InfectoGnostics research campus in Jena, Germany, April 3, 2020. Jens Meyer/AP

Widespread antibody testing could help divide America into two groups: the vulnerable and the recovered. The latter could slowly go back to work, breathing life into the US economy and helping us get back on track before a vaccine becomes available.

But, according to Schaffner, this new study doesn't give cause for much optimism.

"The authors emphasize that there have actually been more infections than anticipated, but I emphasize that this shows there have been many fewer infections than some people have anticipated," he said, adding: "If only 3% of the population has antibodies, that means 97% of the population remains susceptible to the virus."

To reach "herd immunity," the point at which the virus can no longer spread easily because enough people are immune, at least 50% of the US population needs to have had COVID-19.

Ultimately, Schaffner said these results are further evidence that the diagnostic testing that's been done in the US so far isn't enough to give experts a clear sense of the actual case totals.

That's because many people who likely have COVID-19 aren't being tested, and the 25% and 50% of people infected with the virus who show no symptoms probably aren't included in official counts either.

"We need antibody tests to come online to really inform us where we are with this virus," Schaffner said.