It’s months into the coronavirus pandemic and public health officials still don’t know how many people have actually contracted the culprit, SARS-CoV-2. In many countries testing capacity has lagged behind the spread of the virus. Large numbers of people have developed COVID-19 symptoms but have not been tested, and the vast majority of people who had the virus but never developed symptoms and therefore were not tested, are not reflected in official statistics.

Federal and state governments, companies, and research groups are now racing to develop antibody tests to shine a light on the true spread of SARS-CoV-2. While PCR tests currently used to diagnose cases detect the virus’s genetic material, antibody tests can screen for virus-attacking antibodies that are formed shortly after an initial infection. Those antibodies usually linger in the blood long after the virus is gone. One such antibody, or serological, test was given emergency use authorization by the Food and Drug Administration in early April, and a number of other groups are making more tests, and in some cases even deploying them.

The National Institutes of Health has launched a study to detect antibodies in order to gather data for epidemiological models. And a recent survey of residents in a German town was one of the first to use an antibody test among the public, reporting that 14 percent of people there were likely to have been infected with SARS-CoV-2 due to the presence of antibodies.

It’s not clear yet whether milder or asymptomatic cases will develop antibodies.

Policymakers have another reason to scramble to deploy antibody tests: they could indicate whether someone is immune to SARS-CoV-2. With around 3 billion people globally under lockdown, pressure is mounting to re-open national economies. In recent weeks, several politicians have proposed the idea of “immunity passports” or “immunity certificates” to identify people who have had the virus and therefore gained immunity to it and could re-enter the workforce again. Officials in Germany, the UK, Italy, and the US are already discussing such proposals.

The success of such a program hinges on whether everyone who has contracted SARS-CoV-2 actually develops antibodies, whether those antibodies protect against secondary infections, and if so, how long the antibodies hang around in the body. So far, scientists don’t have firm answers to any of these questions. Although antibody surveys of communities around the world could yield information that is crucial to understanding the spread of the pathogen, some consider the idea of “immunity passports” premature.

“People understand that [it] would be very powerful, if we could say, ‘you’re immune now, and you can return to work,’ or, ‘you could safely return to your family if you’re a healthcare worker,’” notes Stanford University and Chan Zuckerberg Biohub immunologist Taia Wang. But there are a lot of unknowns, she cautions. “To get to that point where we’ll know with some certainty what the antibody response means, we just have to collect [more] data.”