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

The billionaire’s space exploration venture, SpaceX, was planning to blast a crewed spacecraft into the sky in May and wanted to stay on schedule. That meant finding a way to keep facilities safely open and limit the spread of Covid-19, a challenge when tests were in short supply.

To monitor the prevalence of the virus among SpaceX workers nationwide, Mr. Musk and the rocket company’s top medical executive worked with doctors and academic researchers to build an antibody-testing program. More than 4,000 SpaceX workers volunteered for monthly blood tests.

This week the group published its findings, which suggest that a certain threshold of antibodies might provide people 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 it doesn’t mean they are going to be immune” to Covid-19, said Galit Alter, a co-author of the study who is a member of the Ragon Institute of MGH, MIT and Harvard. Individuals who experienced fewer, milder Covid-19 symptoms generated fewer antibodies and were therefore less likely to meet the threshold for longer-term immunity, the study found.