Food and Drug Commissioner (FDA) Commissioner Stephen Hahn said on Tuesday that serological tests could help determine who is no longer “susceptible” to coronavirus infection and who can return to work.

The FDA recently granted its first emergency authorization for a rapid antibody blood test, which would allow scientists to determine a patient’s immune response to the coronavirus.

Serological tests use a few drops of blood drawn from a vein to detect antibodies produced by the body to fight off the coronavirus. Doctors and public health officials could use this test to identify a person had been previously infected and potentially recovered from the coronavirus.

Hahn explained on Tuesday:

In other words, the test detects the body’s immune response to the infection caused by the virus rather than detecting the virus itself. In the early days of an infection when the body’s immune response is still building, antibodies may not be detected. This limits the test’s effectiveness for diagnosing COVID-19 and why it should not be used as the sole basis to diagnose COVID-19.

Hahn noted that, along with health data, serological tests could be used to determine if Americans could return to work. He said:

Serological tests can play a critical role in the fight against COVID-19 by helping healthcare professionals to identify individuals who have overcome an infection in the past and have developed an immune response. In the future, this may potentially be used to help determine, together with other clinical data, that such individuals are no longer susceptible to infection and can return to work.

Assistant Secretary for Health at the Department of Health and Human Services (HHS) Brett Giroir said recently that he is “very optimistic” that tens of millions of serologic tests could be ready by May.

Giroir added, “That’s really important as we think about re-opening the country and the economy because if you’ve had the virus and you’ve had an immune response to it, in all probability you are immune and safe from the virus.”

Hahn noted that doctors could also use serological tests to determine if a patient could donate blood plasma, called convalescent plasma, which could be a potential treatment for afflicted coronavirus patients. The treatment would pull blood plasma from recovered patients and inject those antibodies into sick patients.

He wrote, “In addition, these test results can aid in determining who may donate a part of their blood called convalescent plasma, which may serve as a possible treatment for those who are seriously ill from COVID-19.”

“This is why Vice President Mike Pence called on the laboratory community to develop serological tests for COVID-19,” Hahn added.