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U.S. Confronts New Testing Dilemma: How to Figure Out Who Already Had Covid-19

Studies show widely varying reads on how many people have been infected

A laboratory worker runs clinical tests for coronavirus antibodies on a blood sample in Seattle, on April 17.

A laboratory worker runs clinical tests for coronavirus antibodies on a blood sample in Seattle, on April 17.

Photographer: Karen Ducey/Getty Images

Corrected

As the U.S. begins to consider letting people go back to work and lifting social-distancing rules, a new debate over testing is emerging. Researchers are racing to back-fill the gaping holes left by shortfalls in diagnostic tests for currently infected patients, with newly developed tests that can indicate if a person had Covid-19 previously. 

These test, which measure antibodies in the blood and are being used in studies across the U.S. and the rest of the world, are crucial to understanding Covid-19 and the virus that causes it. As they’re rolled out, they can tell researchers and government leaders not just how many people are sick but also how many have encountered the virus at all. That, in turn, may at last give a firmer idea of how deadly the virus is and how many people potentially have some level of immunity.