Antibody tests can't yet say who's immune to COVID-19, but they can show who's at risk

By showing how many people haven’t been infected, the tests provide a picture of who remains vulnerable to the coronavirus.

As antibody tests for the coronavirus become more widespread, a major question still remains: Does a positive test for antibodies mean that a person has developed immunity to the virus?

It may be months before the answer is clear, but that doesn't mean the tests can't yield valuable public health information in the meantime.

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Antibody tests provide information about one thing: whether a person's immune system has developed antibodies needed to fight off a particular infection. A positive test means a person has antibodies, and therefore had the virus at some point; a negative test means a person does not have antibodies and has not yet contracted the virus.

By showing how many people haven’t been infected, the tests provide a picture of who remains vulnerable to the coronavirus.

"There still are people that this virus can infect," Dr. Maria Van Kerkhove, head of emerging diseases and zoonosis unit at the World Health Organization, said during a media briefing Monday. "It's important we remain vigilant."

What remains unclear, however, is whether coronavirus antibodies work well enough, or last long enough, to protect a person from being reinfected.

On Saturday, the WHO caused a bit of global angst among people who are depending on those antibody tests to prove their immunity status — perhaps giving them a pass to return to work or regular social activity — by tweeting that there is not enough evidence that people who have recovered from COVID-19 are protected from reinfection.

The tweet has since been deleted, though the WHO provided a link to the archived thread.

On Monday, the agency clarified what it meant by the original tweet.

"Just because there's no evidence in this area doesn't mean that there's no immunity," Van Kerkhove, said. "It just means that the studies haven't been done yet."

Outside experts agreed. "It's very important to not assume that a positive antibody test means that you're definitely immune, and that you couldn't become infected or that you couldn't spread infection to somebody else," Dr. Marybeth Sexton, an assistant professor of infectious diseases at Emory University School of Medicine, said.