One of the great uncertainties in the coronavirus pandemic is how many people have been infected without knowing it or showing symptoms. Fortunately, there are now tests for that, and broadly deploying them will be critical to saving lives and getting Americans back to work.

Dozens of commercial and public health labs around the world are rolling out antibody tests that can show if an individual was recently infected with the virus and has developed immunity. When fighting a pathogen, the immune system produces proteins known as antibodies that bind to specific molecules known as antigens on the invader’s surface like a lock and key.

Antibodies circulate in the blood for weeks and even months after infection, providing resistance to another onslaught, though the immune system remembers how to produce them on demand. Vaccines work by instructing the immune system to make antibodies to shoot down pathogens before they proliferate.

Because antibodies linger in the blood, they are especially useful for determining if someone has been infected with the coronavirus. Some experts estimate that more than half of infected individuals show mild or no symptoms, though the true figure could be higher. Around three-quarters of people infected with flu viruses show mild or no symptoms.

Researchers and governments are now seeking to administer antibody tests to large groups of people to better gauge the virus’s prevalence in the general population. This would provide a more accurate assessment of lethality and allow exposed health-care workers with immunity to stay on the front lines.