Sonoma County using antibody tests to help determine scope of coronavirus outbreak

The tests detect often unknown previous coronavirus infections, and the results will help guide the next stage of response to the ongoing battle against the virus.|

After three months into the local fight against the new coronavirus, it still remains unclear how many Sonoma County residents actually have been infected by the contagion since only a sliver of the county’s nearly 500,000 people have been tested.

In fact, just 32,000 people, or 6% of the population, know for sure if they’ve had or still have the virus because they tested positive. Since the county mounted an aggressive, multifacted testing regimen beginning in April, more and more cases have emerged but the area’s total infection rate continues to hover at only 2% of those tested.

With the COVID-19 outbreak under control at least for now, county public health officials are turning their attention to trying to discern how many residents were infected by the virus, yet didn’t likely know it because their bodies fought it off. To get a clearer picture of the magnitude of the virus spread, county health officials last week started antibody testing approved by the U.S. Food and Drug Administration. These tests detect often unknown previous coronavirus infections, and the results will help guide the next stage of public health response to the ongoing battle against the highly contagious pathogen in the community. And the test results could possibly point to the need for more public health precautions, said Dr. Sundari Mase, the county’s health officer.

“If (for example) 5% of our community is already infected, we’d tell people we didn’t know that this extra huge number of people got infected so be careful, wear your facial coverings, really observe the mitigation measures,” Mase said in an interview this week.

What’s more, Dr. Lee Riley, professor of infectious diseases and vaccinology at UC Berkeley School of Public Health, said if the epidemic continues and a second wave of infections occurs in a community, local health officials can evaluate whether people who have the antibodies showing previous infections are less likely to get infected or develop symptoms during such a spike or surge.

Antibodies are proteins that help fight off infections. But it’s not clear yet if having antibodies to the coronavirus can protect someone from becoming reinfected with the infectious disease or how long that antibody protection may last for an individual.

County health officials are prioritizing the tests for COVID-19 antibodies in individuals that are part of high-risk groups, such as first responders, those who previously tested positive for the coronavirus and the people they’ve come in close contact with. Infected residents are being tested to determine if they in fact developed antibodies in their bloodstream, while their contacts are being tested to see if any previously were misdiagnosed negative for the infectious disease that has swept the globe, killing at least 113,000 people in the United States, nearly 4,900 in California and four residents of Sonoma County.

If the antibody test results point to a far higher prevalence of infection in the county than previously thought, public health workers could begin testing the general population, Mase said.

It’s not prudent to conduct widespread antibody testing among the general public now, she said, because it could be that the “true percentage” of county residents who actually have contracted the virus could be less than 1%. When that real prevalence, the share of residents in a community infected by the coronavirus, is less than 1%, the antibody testing can result in a number of false positives or false negatives, the county health officer said.

Meanwhile, Vitalant, operator of the area’s major blood donation center, also has begun conducting COVID-19 antibody tests on everybody who donates blood. It take two weeks for donors to get the results.

The new testing at the blood bank, which began June 1, is aimed at letting blood donors know whether their immune system has produced antibodies to fight the coronavirus, though that does not mean someone is definitely immune to it. Such donors also could be eligible to donate so-called “convalescent plasma,” an experimental therapy that could help people recover from severe cases of COVID-19.

Unlike the county’s ongoing diagnostic testing confirming people have the virus, this test only detects antibodies that point to a previous viral infection. And the test reveals if the immune system indeed has responded to defeat the infection.

Mase said antibody testing could be used to retest individuals who previously were given diagnostic tests through the county’s contact tracing program because they were in close contact with someone infected with COVID-19. Some of these individuals could have tested negative earlier, because they were tested before the virus could be detected in their bodies.

“If they were asymptomatic we could have missed them, so it would be interesting to see how many that were negative on the (diagnostic) test are positive on the antibody test,” Mase said.

The county began using its new Italian-made DiaSorin Liaison XL antibody test-processing equipment on Saturday and plans to conduct tests daily. Health officials are initially testing paramedics, firefighters, law enforcement officers, as well as the roughly 670 county residents who have been diagnosed with COVID-19.

The county’s goal is to conduct about 110 antibody tests a day, with the target of testing 3,500 people in the next four weeks. The test is free and people can expect results within two weeks. County public health investigators will contact people for the tests and make appointments. Since conducting 94 tests on the first day, the county has done 390 antibody tests as of Wednesday.

At Vitalant blood donation centers, the new antibody testing program is resulting in many more people interested in giving blood, Vitalant spokesman Kevin Adler said, noting the test is the latest addition to the battery of tests performed on donated blood. Donors’ personal information and individual test results are kept confidential.

“We’ve had people show up and say, ‘I’m here because I want to find out my antibody status,’?” Adler said.

He said plasma taken from someone who previously had COVID-19 can be used to treat coronavirus patients suffering with severe conditions in the Bay Area. In order to give the convalescent plasma, blood donors must first have a confirmed COVID-19 diagnosis, but also must have a second test that assures they no longer have the virus, he said.

Mase said that particular COVID-19 medical therapy using blood plasma from previously infected people is not in high demand in Sonoma County because there are few people hospitalized with serious cases of the coronavirus. However, Adler said the plasma collected in Santa Rosa could be used at any one of 45 hospitals Vitalant serves throughout the Bay Area. The blood collection center began collecting and distributing convalescent plasma in April.

Vitalant officials are hoping to get referrals from county health officials for blood donations and antibody tests, and the blood bank may arrange to share aggregate data of all its antibody test results with the county.

Both county health officials and Vitalant said their antibody tests have a high degree of accuracy and received FDA approval. County health officials said with the antibody test kits made by Italian diagnostics group DiaSorin, nearly 90% of those who test positive for antibodies indeed have COVID-19 and nearly 100% of people who test negative never were infected by the virus.

Riley, the UC Berkeley infectious diseases professor, said ultimately antibody testing could provide useful information about the effectiveness of a future coronavirus vaccine.

“When they try to develop a vaccine, if the vaccine induces the same type of antibody response - the antibody response is protective against new infections - that would be a good indication that vaccines will work,” he said.

You can reach Staff Writer Martin Espinoza at 707-521-5213 or On Twitter @pressreno.

UPDATED: Please read and follow our commenting policy:
  • This is a family newspaper, please use a kind and respectful tone.
  • No profanity, hate speech or personal attacks. No off-topic remarks.
  • No disinformation about current events.
  • We will remove any comments — or commenters — that do not follow this commenting policy.