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After weeks of warning that treatments and vaccines for COVID-19 could be months away, scientists are showing enthusiasm for one line of attack that’s not so distant: widespread testing for the presence of antibodies. These “serology tests,” as they’re called, could indicate a past infection — and some degree of protection against getting the coronavirus again.

That’s important because scientists know that the 1.5 million cases worldwide represent only the tip of the iceberg. There are many more people who never got sick enough to seek out a test. In theory, antibody testing could identify those people and give them a sort of COVID-19 license or passport to re-enter the workforce, or help out in hospitals and labs battling the disease.

But a positive antibody test is not an automatic ticket back to work. “People are thinking about this wrong,” says Harvard pathology professor David Walt, who is developing new COVID-19 tests of different kinds. Eventually it could work that way, but not until people can also be tested to ensure they don’t have any active virus lingering in their bodies. We still lack the quick, accurate tests we need to pull that off on a large scale.

Nonetheless, several promising, smaller-scale studies are in the works. Population-wide studies are planned in San Miguel County, Colorado, and the Veneto region of Italy. In the latter, scientists plan to start testing health care workers next week. And a massive study in the United States is expected to give results soon from testing thousands of samples of donated blood to see how many people might have had under-the-radar cases in six cities including Boston, New York and Seattle.

As people eventually emerge from social distancing, public health researchers can use both antibody tests and the more standard tests that pick up viral genetic material to trace where the virus is being spread and alert people who might have been exposed before they spread it further.

But right now most antibody tests — there are more than 50 already developed — aren’t up to the task of screening whole cities, says Harvard’s Walt. “Most tests are low-throughput technologies, so you have to run one sample at a time,” he says. But high-throughput tests are being developed, and they should be used on health care workers first, he says. That alone will involve millions of tests a week.

Another reason why we won’t be seeing back-to-work passports anytime soon is that some people who test positive for antibodies may still be infectious. It takes a few days after infection for people to develop antibodies, Walt says, so there could be a period of a few days or even weeks during which they have both antibodies and enough live virus to infect others. And while it seems likely that post-infection immunity would last a few months, scientists can’t yet be sure.

Scientists also need to better understand what’s going on with asymptomatic cases. It’s even less well understood when and for how long such people might be infectious, and whether asymptomatic cases would mount enough of an immune response to certify someone as safe from re-infection.

False positives could also be a problem. There’s a risk that tests will pick up other antibodies, including ones from more common coronaviruses, like those that cause the common cold.

One promising solution is a test called VirScan, developed in 2015, with the power to detect antibodies to thousands of human viruses using a single drop of blood (the sort of thing disgraced biotech Theranos promised but never delivered). The developer of the test, Harvard professor Stephen Elledge, a geneticist who focuses on immunology, says he’s already adapted it to pick up multiple antibodies to the novel coronavirus, so it’s very unlikely to get a false positive.

He says the test can also be useful in advancing the use of antibodies from recovered patients as a treatment for those who are critically ill. A number of researchers see promise in this “convalescent serum” treatment. And he says his test can help vaccine developers make sure they have the right antibodies.

Ultimately, he says he hopes antibody testing on banked blood will shed light on how this pandemic got started. It’s possible, he says, that the virus jumped to humans months before it started showing up around a seafood market in Wuhan as the virus we’re contending with today.

And, yes, he hopes his technology or something like it will be used in a few weeks to send people back to work. “Virtually everyone in the United States is going to want to know if they’ve had the virus,” he says, “so they can go live their lives again.”

Faye Flam is a Bloomberg columnist.

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