“This new test just tells you that you’ve been exposed to coronavirus, but doesn’t prove that you have immunity,” said Dr. Angela Rasmussen, a virologist at Columbia University. “The next generation of antibody tests, which assay neutralizing antibodies, can tell us who is actually immune.” These tests may be months away from development.

Nonetheless, the current test has valuable benefits. The most obvious is that it does establish that people have been exposed, and once they are truly asymptomatic, it is unlikely they can infect others — so they can safely venture outside. This means they can return to work, which will be especially important for health care workers, who are at the highest risk of exposure.

(There are stories of people testing positive for the virus and getting it again but little scientific evidence for this. It could be that some of these reports are of people who had a false-positive viral test and had a simple cold, and then subsequently contracted the coronavirus.)

Another benefit is that it could be used as a fast if imperfect screen for coronavirus infection itself, Dr. Rasmussen told me. Unlike the nasal swab test for the virus, which is currently limited to severely ill patients, the antibody test could be given to anyone who presents with mild symptoms. (Though a positive result would need confirmation by the more definitive virus test.)

It would also be useful to identify asymptomatic people, who can unknowingly spread the disease. According to the Centers for Disease Control and Prevention, as many as 25 percent of people with the coronavirus may be asymptomatic. Widespread antibody testing would give us a much better understand of the scope of infection.

Last, testing could help those who are still ill. It is going to take time for us to have an effective vaccine or drug for this scourge. But people who have survived coronavirus infection may be walking around with a treatment in their bodies. If their blood is found to contain sufficient levels of neutralizing antibody, they could donate their so-called convalescent plasma to patients who are severely ill. The theory is that the donor’s antibodies could help block the virus from entering the recipient’s cells.

Whether plasma treatment works for the coronavirus has yet to be proved, but it has been helpful for other viral illnesses like H1N1 (swine flu) and SARS. Last week, the F.D.A. allowed the investigational use of convalescent plasma for patients with the coronavirus.