Swiss health-care giant Roche Holding is adding its heft to efforts to develop an antibody test for the new coronavirus with plans to launch a product in early May.

Antibody tests are seen by many governments around the world as key to better understanding the spread of mild and asymptomatic cases of Covid-19. They search for specific antibodies—tailormade proteins made by the body in response to infection—as a sign that a person has encountered, and recovered from, the virus. They are different from the swab tests used to diagnose a current infection.

Roche, the world’s biggest diagnostics company by revenue, is one of dozens of groups developing antibody tests but its size means it can scale up quickly. The company will be able to churn out test kits numbering in the “high double digit millions” a month, according to Thomas Schinecker, who leads Roche’s diagnostics business. The tests, made in Germany, will run on the company’s diagnostics machines that are already found in labs around the world.

A number of commercial antibody tests from smaller companies are already available but many aren’t deemed accurate enough by governments. Mr. Schinecker said he was confident that Roche’s test, which is still under development, would clear the bar. Roche plans to seek emergency-use authorization from the Food and Drug Administration. “Most of the known players will make sure they come with a test that is very precise,” he said.

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