How far has coronavirus spread? This test is a first step in trying to get back to normal life

Antibody test for the coronavirus
A scientist in a German lab presents a test for antibodies to the coronavirus. A new study conducted by L.A. County will use antibody tests to track the prevalence of the coronavirus.
(Jens Meyer / Associated Press)

It has been one of the most glaring unanswered questions of the coronavirus crisis: How much has the virus spread? A new study in Los Angeles County might help provide some answers.

Starting Friday, county health officials will begin testing the blood of 1,000 randomly selected residents, including those with no symptoms, to see if they have or had COVID-19. Using emerging technology that tests for antibodies to a virus, the study has the potential to shed light on the true mortality rate of the coronavirus, the efficacy of social distancing efforts and when this unprecedented clampdown on daily life could end.

Health officials and researchers say the results of this effort, known as serological testing, could paint the most complete picture yet of the sweep of the pandemic in the nation’s most populous county.


“Is it 1% of our population? Is it 10% of our population? That’s the difference between 80,000 adults and 800,000 adults. We have no idea,” said Dr. Paul Simon, chief science officer of the Los Angeles County Department of Public Health.

With the study’s launch, the county joins the growing ranks of governments and researchers that are turning to antibody tests as a new frontier in the campaign against COVID-19.

Antibody testing has already begun in Santa Clara County, with researchers drawing more than 3,000 people’s blood over two days last week at three drive-up testing sites. The Centers for Disease Control and Prevention began last week testing the blood of people living in coronavirus hot spots. And Thursday, German health officials announced plans to run Europe’s first large-scale antibody testing program.

So far most testing has been done with nasal swabs, meant to detect the presence of the virus. But the pool of those tested has skewed toward the very ill, as health officials have put the highest-risk patients at the front of the line.

With the new blood tests, scientists hope to trace antibodies, which the body develops to fend off the coronavirus and linger after it is gone.

The presence of those antibodies probably offers some protection against reinfection, although it is not yet known the extent to which people would be immune from the virus.


The research aims to fill in a number of blanks about COVID-19, including how deadly the disease truly is — a figure that is difficult to pin down without knowing how many people have the virus, said Neeraj Sood, a professor and vice dean for research at USC’s Price School of Public Policy, who worked with the county on the study.

At this point, the county reports a death rate of 2.8% of people with confirmed cases. By contrast, the average death rate of the seasonal flu is 0.1%

“If the mortality rate of COVID is 10 times that of the flu, then we should all be freaking out and we should be staying at home and making sure that we practice very strong physical distancing,” Sood said. “But if the true mortality rate of COVID is five times less than the flu, then we don’t need to be doing that. Then it is a less deadly disease.”

Using antibody testing over time could also illuminate the effects of the aggressive physical distancing measures in Los Angeles and across most of the country — a step that has brought nearly all economic activity to a halt. If the rate of infection continues to climb in the population even with these measures in place, it may prompt politicians to rethink their usefulness in the face of massive job losses.

Additionally, the tests could offer clues as to when at least 60% of the population has at one point been infected, achieving “herd immunity,” which would significantly thwart the spread of the virus.


Experts caution that serological testing is not a panacea. It is still unclear how much immunity a person has from the coronavirus when antibodies are present. And the surge of new tests coming to the market means there is a wide variance in accuracy and reliability.

This week, Oxford University researchers found that at-home blood tests used in the U.K. were faulty, after the British government had ordered more than 4 million of such tests for residents.

“There are a lot of unknowns,” said Kevin de León, a former state Senate leader and incoming L.A. city councilman who has helped with the L.A. study’s launch. “However, we have this opportunity to scientifically determine whether there are those among us who can reenter the workforce safely, and we’ve got to seize it.”

So far, it has been impossible to quantify how many people have had the virus, because of a shortage of nasal swab test kits. The dearth of information threatened to hamper hospitals’ ability to prepare for the virus last month, as the wave of cases began to swell.

“I felt that we were flying blind,” said Dr. Brad Spellberg, chief medical officer at Los Angeles County-USC Medical Center. “We had no idea what was going on in the community around us.”

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In mid-March, he decided to perform nasal swab tests on roughly 130 patients who came to his center’s emergency room and urgent care with mild flu-like symptoms. It offered a glimpse into a population beyond the highest-risk, most severe cases that have been the main focus of testing so far. He found that 5.3% of the patients with milder symptoms tested positive for COVID-19, a result he called “alarming.”


“These were young, active people,” Spellberg said. “They were going to work. They were going to community events. They had no idea that they had coronavirus and they had the real potential to be spreading this.”

Spellberg said the results pushed him to escalate his planning for needed personal protective equipment and hospital beds. The county also took notice.

Gov. Gavin Newsom has repeatedly said California needs to expand beyond diagnostic tests of sick patients to understand the virus.

At a news conference Thursday, Newsom said antibody blood tests are “critical in terms of our capacity to make a determination about community spread and about immunity.”

The governor has said testing broader samples of the public throughout the state, not just people exhibiting COVID-19 symptoms, will help officials determine when to lift the stay-at-home order. But that kind of testing is occurring only at the local level or at universities.


The blood testing conducted last week in Santa Clara County involved participants who were found via social media.

“This was the first-of-a-kind, large attempt to measure how many people in a large county have been infected,” said Eran Bendavid, a specialist in infectious disease and professor at the Stanford Health Policy program, who is leading the project in conjunction with the county public health office.

The first round of results, which have been collected but not yet published, makes clear that many people have been infected and are not showing symptoms, Bendavid said.

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Stanford University on Wednesday also began offering widespread serology testing to anyone who wants it. Dr. James Zehnder said that the university medical system has the capacity to perform up to 750 serology tests a day and expects to increase capacity soon.

Currently, Zehnder said, there is no backlog of testing and rules have been relaxed so that any person who wants a diagnostic or serology test can be accommodated.

In Los Angeles, the research team will stage six drive-through testing sites throughout the county Friday and Saturday. Participants were invited through a marketing firm to create a random sample that mirrors the demographic makeup of the county, in order to best allow the data to be extrapolated to apply to the county as a whole.


The kit functions somewhat like a pregnancy test. Participants supply a drop of blood via pinprick; when combined with a buffer solution, a number of lines appear to indicate the presence of certain antibodies. Simon, the L.A. County chief science officer, said that individuals can get their results on an online portal within 24 hours and that the county would make its analysis of the data publicly available late next week.

The antibody tests were secured by Sood for free through a donor. The marketing firm Lieberman Research Worldwide is working pro bono. Some funding and logistical support is coming from the USC Schwarzenegger Institute for State and Global Policy and officials are raising money from private donors to repeat the study every two weeks for three months, which Simon estimated would cost at least $1.1 million.

Times staff writers Anita Chabria and Taryn Luna in Sacramento contributed to this report.