Elon Musk got 4,000 SpaceX workers to join a COVID-19 study. Here's 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 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.

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"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.

The idea is one that other researchers are exploring as they and public-health officials try to understand Covid-19 immunity.

"To really nail this down at a public-health level would require doing reinfection studies and following people for reinfection" over time, said Joshua T. Schiffer, associate professor at the Fred Hutchinson Cancer Research Center's vaccine and infectious-disease division.

As vaccines roll out slowly across the globe, the scientists who studied SpaceX workers say their findings could be used to inform who is most vulnerable to the virus and should be vaccinated first. For example, those with no antibodies in areas with high case counts could get priority, Dr. Alter said.

Companies from Alphabet Inc.'s Google to Goldman Sachs Group Inc., Amazon.com Inc. and Tyson Foods Inc. offer Covid-19 diagnostic tests to get a moment-in-time snapshot of who is infected. Few businesses have regularly tested worker blood samples for antibodies.

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Representatives for Space Exploration Technologies Corp., the official name of the closely held Southern California company, declined to comment on the testing.

Mr. Musk, who is chief executive officer of SpaceX and of Tesla Inc. and has decried shutdown orders, took a personal interest in the research and had the scientists brief him and top SpaceX executives during the pandemic on how antibodies and vaccines work, Dr. Alter said.

Mr. Musk in November said he tested positive for the virus. "Mild sniffles & cough & slight fever past few days," he tweeted at the time.

Dr. Alter, who studies immunology and the molecular mechanisms of how antibodies fight diseases, created at the start of the year high-throughput Covid-19 antibody testing. Her work attracted investments from the hedge-fund manager Nancy Zimmerman, former Soros Fund Management CEO Mark Schwartz and his wife, Lisa Schwartz, as well as a host of philanthropies (among them the Musk Foundation) and government agencies.

In April 2020, when Covid tests were scarce, SpaceX contacted Eric Nilles, an infectious-disease expert at Harvard, and he enlisted Dr. Alter's help.

Together with SpaceX's medical director, Anil Menon, they built a testing program. SpaceX recruited workers from California to Florida who were willing to have their blood tested monthly starting in April.

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Before the pandemic, Dr. Menon had set up medical facilities at SpaceX worksites across the country. SpaceX has sent astronauts and aims to eventually send civilians into orbit. He used some of those facilities to quickly scale up blood-drawing stations and recruited medical interns from local hospitals to collect samples.

In May, SpaceX launched a successful test flight of its Dragon capsule in Florida carrying two astronauts.

In June, samples Dr. Alter processed from local workers foretold worsening cases in Texas. Instead of a typical 3% positivity rate for the virus, 12% of the samples suggested infection. She reran them multiple times and confirmed that they were correct, which led the company to send infected workers home and advise them to isolate.

Of the roughly 4,000 SpaceX workers tested multiple times, 300 became infected with Covid-19. Researchers had enough data on 120 people to dig deeper into their infections and subsequent levels of antibodies to draw conclusions in the study.

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The median age of that small sample was 31, and 92% of them were male, which the authors acknowledge might skew their findings because people of different ages and backgrounds present different immune-system responses. The study included test results between April and June.

"The good news is most of the vaccines induce [antibody] levels way higher than these levels" for people who get both doses, Dr. Alter said. "So far it is pretty clear that we are hitting levels that are orders of magnitude higher with vaccination."

The Centers for Disease Control and Prevention has said that having antibodies may provide some protection, but it is unknown how long it may last. Researchers and diagnostic companies are working to understand what level of antibodies confers immunity.

"It would be great to have a clear numerical cutoff to say...above this level [of antibodies] you're protected, below you're not, but we don't have such a cutoff right now," said Philip Dormitzer, Pfizer Inc.'s viral-vaccine research-and-development chief.

SpaceX and its research partners continue testing each month and are now monitoring for reinfection, particularly as mutations spread, researchers involved in those efforts said. So far, they have observed some reinfection among workers who were found to have low levels of Covid-19 antibodies in past testing.

In November, the company launched four astronauts into orbit as part of its first operational mission with humans on board.

--Daniela Hernandez contributed to this article.