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What can coronavirus antibody tests actually tell us?

Assuming that positive antibody test results confer immunity is a "very dangerous conclusion," say experts.
Image: Antibody testing
Medical laboratory scientist, Alicia Bui, runs a clinical test in the Immunology lab at UW Medicine looking for antibodies against SARS-CoV-2, a virus strain that causes coronavirus disease (COVID-19) on April 17, 2020 in Seattle, Wash.Karen Ducey / Getty Images

Results from antibody testing studies for the coronavirus are starting to paint a picture of the scope of the disease it causes in parts of the U.S.

But the initial findings won't identify which people are immune to the virus; in other words, a positive result from an antibody test can't be considered a "get out of jail free" card for those who wish to stop social distancing and get back to life as usual, experts say.

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"There is not going to be a magic wand that will say you can be out of this crisis," said Neeraj Sood, a professor of public policy at the University of Southern California.

Sood released results this week on COVID-19 antibody testing of 863 adults living in Los Angeles County. The participants were selected, he said, to reflect a population representative of the county.

The results are considered preliminary, as they haven't been published in a peer-reviewed medical journal. Still, the study is one of the country's first to give a glimpse into what may be the true numbers of COVID-19 infections.

The USC report estimates that 4 percent of Los Angeles County residents have COVID-19 antibodies, meaning they either had an active infection or have recovered. Experts suspect that people who've had the virus can't immediately become infected a second time, although it's unclear how long the protection lasts.

Sood said the 4 percent figure translates to about 320,000 people in the county — far higher than the actual number of positive COVID-19 tests, which was about 8,000 at the time the research was conducted in early April.

Image: COVID-19 blood test for antibodies
Medical laboratory scientist Alicia Bui runs a clinical test in the immunology lab at UW Medicine in Seattle looking for antibodies against the coronavirus on Friday, April 17, 2020.Karen Ducey / Getty Images

While the USC estimate may seem high, it actually suggests that Los Angeles County may be in the early phases of the pandemic.

If "roughly 4 percent of adults have been infected, it means the remaining 96 percent are still susceptible to this disease," Sood said. He estimated that 60 percent of the public would need COVID-19 antibodies for what's known as herd immunity to kick in, meaning enough people in a community have a level of immunity that makes it more difficult for infectious diseases to spread.

It's unclear whether the 4 percent figure would hold true for other areas of the country. A similar antibody study from Santa Clara County in Northern California estimated that 2.5 percent to 4.2 percent of those residents have antibodies. That research also wasn't peer-reviewed.

What do COVID-19 antibodies mean?

If a person has a positive test for COVID-19 antibodies, it means the person has been infected with the virus at some point in the past.

"Just because you're antibody-positive doesn't necessarily mean you're immune," said Joel Baines, a virologist and professor of pathobiological sciences at Louisiana State University. "That's a very dangerous conclusion."

Only time — perhaps a year or more, experts predict — will be able to determine whether immunity lasts several weeks, several months, several years or even a lifetime.

The unknowns suggest that the public shouldn't rely on antibody tests to determine when to resume normal activity.

"It's risky to base everything on that," Baines said.

Lower mortality rates

But the USC data do offer some reassuring news: COVID-19 death rates may be lower than expected. When experts factor in those additional cases, the mortality rate plummets to less than 0.2 percent. That's because there are more patients overall, making the number of people who've died a smaller percentage of cases in total.

USC researchers will repeat the study with a different set of Los Angeles County residents every few weeks over the coming months to give public health officials a better sense of how the virus is spreading in that community. Similar testing should be done in a variety of areas around the country, Sood said, for a better understanding of how the virus is acting nationwide.

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Still, antibody tests are only one piece of the puzzle for reopening the country. Diagnostic tests and contact tracing capabilities are essential.

"It will be critical to have a system in place in every major city where in the workplace you can get quickly tested and then have a system in place do the contact tracing," said Dr. Peter Hotez, co-director of the Center for Vaccine Development at Texas Children's Hospital and dean of the National School of Tropical Medicine at the Baylor College of Medicine in Houston.

Sood agreed.

"This pilot study was to show — look, this can be done," he said. "We should be doing this on a much larger scale."

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