The U.S. Centers for Disease Control and Prevention (CDC) has recently started testing for coronavirus exposure in asymptomatic people using serological tests—a finger prick blood test to see if someone has recovered from the virus by identifying antibodies in their blood. This differs from the nasal swab PCR (polymerase chain reaction) tests that are being used at drive-through sites across the Philadelphia region to diagnose those who currently have the virus.

Serological testing could identify people, most notably health-care workers, who could safely go back to work. It can also identify those who don’t have immunity so they can be first in line for a vaccine, which won’t be available for at least another year.

Last week, the U.S. Food and Drug Administration issued its first emergency use authorization for a serological test that can be done in certified labs. Other healthcare companies are seeking government clearance for home kits that allow users to submit a scanned picture of their blood to doctors via their phones.

“We definitely want to know if people have antibodies to provide immunity, especially as we’re waiting for a vaccine,” said Kathleen Bachynski, an assistant professor in public health at Muhlenberg College. “That information is going to be crucial to inform our understanding of public health risk and the course of the pandemic.”

But before you search out serological testing so you go back to work or start attending public gatherings again, know there are many caveats to this type of test.

Infectious diseases experts have cautioned that taking a serological test immediately after an infection may result in a negative result because the immune system has not had time to develop antibodies. Most people become positive for antibodies about 14 days after they start experiencing symptoms. People can also test positive if they have antibodies against a different coronavirus, like those that cause the common cold.

And while the tests can identify whether antibodies exist, there is no way to know how well those antibodies are working, health officials say. It’s also unclear whether an immune response can protect people who are exposed to larger “doses” of the virus, like health-care workers.

Some countries, like Germany, are considering issuing “passes” to people who test positive for COVID-19 antibodies that will allow them to go back to work. But those steps may be premature, as researchers don’t know how long immunity lasts, and they can’t use serological tests to find out whether people who test positive for antibodies are still contagious.

“We still have a lot to learn about how immunity works and how accurate these tests are,” Bachynski said.

Given all of the unknowns, experts say the best approach, regardless of whether you think you’ve been infected, is to follow social distancing measures to avoid getting infected until a vaccine is approved and deployed.

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