Elon Musk asked 4,000 SpaceX workers to join a Covid-19 study. Here is what he learned.

When Covid-19 shut down the U.S. economy in March, Elon Musk had a rocket to launch.

The billionaire’s space exploration firm, SpaceX, planned to detonate a crewed spacecraft in the sky in May and wanted to stick to the schedule. This meant finding a way to keep facilities safe and open and limit the spread of Covid-19, a challenge when testing was scarce.

To monitor the prevalence of the virus among SpaceX workers across the country, Musk and the rocket company’s senior medical officer worked with doctors and university researchers to develop an antibody testing program. More than 4,000 SpaceX employees have volunteered for monthly blood tests.

This week, the group published its results, which suggest that a certain threshold of antibodies could provide people with lasting protection against the virus. Mr. Musk is listed as a co-author of the peer-reviewed study, which appears in the journal Nature Communications.

“People can have antibodies, but that doesn’t mean they’re going to be immune” to Covid-19, said Galit Alter, co-author of the study who is a fellow of the Ragon Institute of MGH, MIT and Harvard. People who had fewer symptoms of milder Covid-19 generated fewer antibodies and were therefore less likely to reach the threshold of long-term immunity, according to the study.