The U.S. Food and Drug Administration has authorized the first test for coronavirus that measures antibodies in the blood. Such tests could identify people who have recovered from COVID-19, a key for knowing who's immune and for developing a vaccine.

Cellex Inc. of Research Triangle Park, North Carolina, received an emergency use authorization for its test Thursday from the FDA.It uses blood drawn from a vein to measure antibodies to the SARS-CoV-2 virus that causes COVID-19.

It is the first of an expected flood of such tests as companies race to develop them. Antibody tests are already being used in several countries, including China, South Korea and Singapore.

“Antibody positivity likely means a person has recovered and can't be reinfected. This test will be extremely valuable, especially for healthcare workers,” said Alan Wu, a professor of laboratory medicine at the University of California, San Francisco and chief of the clinical chemistry and toxicology laboratories at Zuckerberg San Francisco General Hospital. His team is testing several possible COVID-19 blood tests.

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The Cellex test and other blood-based tests are different from nasal swab tests that identify patients with active SARS-CoV-2 virus in their mucous membranes. Those tests show a patient has an active case of COVID-19.

Blood tests can determine someone had — and recovered from — COVID-19. More than 30 companies have similar tests in various stages of readiness.

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Such tests cannot come too quickly for doctors, who see them as a way to confirm which staff members have developed possible immunity to the virus.

“We could sort out who among the health care workers has antibodies and assign them to coronavirus patients. They’d still use protection, but would have a much greater sense of security,” said William Schaffner, a professor of preventive medicine and infectious disease at Vanderbilt University in Nashville, Tennessee.

Such tests also will be crucial for vaccine production. To prove a vaccine works, you must show antibody production in someone who hasn’t been exposed to COVID-19. Only an antibody test can do that.

How the tests detect coronavirus

The tests look for two antibodies in the blood, immunoglobulin M (IgM) and immunoglobulin G (IgG.)

Antibodies are the body’s way of remembering how it responded to an infection so it can attack again if exposed to the same pathogen. IgM antibodies are the first line of defense and appear within several days of the infection. IgG antibodies come later, as the body is clearing up the infection.

While IgM and IgG fight all kinds of infections, the blood tests for COVID-19 look for protein particular to the virus so they know the body is producing antibodies to the coronavirus and not, for example, the seasonal flu.

The speed at which the tests are being considered has been tremendous, with the Food and Drug Administration fast-tracking emergency use authorizations for companies.

“Normally this would take months or years to get a test approved. But because of the emergency nature of the pandemic, we’ve gotten them quickly,” said Wu.

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Why antibody tests are important

Antibody tests can identify people who had COVID-19 with no symptoms, a group that may be much larger than realized.

“There’s some data out there that 15% to 16% of kids have had asymptomatic infections. They could be the secret spreaders. But without tests, we don’t know,” said Mark Slifka, a professor of viral immunology at Oregon Health and Science University in Portland, Oregon.

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Doing broad testing of even a portion of the population in the coming months could also give a sense of what percentage of people was infected this year. That could predict who might be immune if the virus comes back again next fall, said Silka.

Immunity to viruses can last from months to a lifetime. Schaffner expects having had COVID-19 once would likely give “pretty solid protection for a year.” But he cautioned predictions for a newly emerged virus are notoriously difficult to make.

“We don’t know that this new coronavirus has read the textbook and knows what it’s supposed to do,” he said.