TOKYO—Jon Rahm keeps testing positive. 

The first set of bad news came after the third round of the Memorial Tournament in June, when Rahm walked off the course with a six-shot lead only to find out a Covid-19 test he had taken came back positive. The price of the result was the $1.7 million payday he was near certain to earn. 

This week, the 26-year-old Spanish golfer’s quest for Olympic gold was derailed once again by the same opponent: the Covid-19-causing coronavirus. Rahm tested positive on the third test he was required to take before traveling to Tokyo for the games.  

The surreal sequence of events was made even more surprising by one other detail: Rahm had been vaccinated. 

“I would have loved to have been the first Spanish Olympic gold medalist in golf, but unfortunately destiny had other plans,” he wrote on Twitter in Spanish and English. “This is a great reminder for all of us that we’re still in a pandemic.” 

It may also be a great reminder that breakthrough cases, or infections after being fully vaccinated, aren’t all that rare, scientists say, especially with variants like Delta making the rounds. Some research suggests that Delta is slightly better at evading currently available vaccines than other versions of the virus.

If his positive case is confirmed, Rahm’s experience may be a preview of what’s in store for the rest of the world—and particularly the sports world—as Covid remains a fact of life.

“You’re going to have this over and over again,” said Stanley Perlman, a University of Iowa coronavirus researcher and a member on the panel advising the FDA about authorizing Covid-19 vaccines. “This is a story more about being cautious and being sad for the golfer than it being any risk for the world.” 

While Covid-19 vaccinations are highly effective at preventing hospitalizations or death from the virus, they’re not foolproof in preventing infection. This poses problems for events like the Olympics and raises broader questions about immunity in the long term. Photo: David Crigger/Associated Press

Covid reinfections, like breakthrough cases, tend to be less severe. They can be prevented by the same mitigation strategies that have been scientifically shown to work for infections generally, like masking and ventilation. Vaccines currently authorized in the U.S. for Covid-19 are highly effective at preventing severe disease, hospitalizations and deaths, but they were not designed to totally prevent infections. Other highly effective vaccines, like those for measles, work similarly. 

Researchers don’t yet know for sure whether people who get Covid breakthrough cases are contagious, but some data suggests that they carry around less virus, which would lower their chances of spreading it to other people.

Dr. Perlman and others said that if indeed Rahm caught Covid again, it was unlikely he posed a real threat to others. 

Rahm had received the one-dose Johnson & Johnson vaccine before testing positive at the Memorial. But he was still within the two-week incubation period it takes for the vaccine to realize its full effects. 

“Two weeks is just when your antibody levels start going up,” said Theodora Hatziionnou, a Rockefeller University virologist studying Covid-19 immunity. “That’s when you can detect them very, very nicely. But it’s also not the peak. They continue to grow for a few weeks.” 

The long-lasting efficacy of the Johnson & Johnson vaccine in particular has been put under the microscope lately, especially in relation to the Delta variant. Yet the Johnson & Johnson vaccine has been notably popular among athletes in recent months. Their schedules require extensive travel, and scheduling one dose can be easier. It also takes effect sooner than the two-dose regimens from Pfizer and Moderna. 

Rahm’s story—testing positive twice after receiving the vaccine while winning the U.S. Open in between—is particularly strange. But the concern for sports is that soon it may not be a terrible outlier. 

“It is possible that the first infection was well controlled by emerging vaccine-elicited immunity, and therefore the first infection might have had little immunological consequence itself,” said Paul Bieniasz, a Rockefeller University virologist who’s also working on Covid-19 immunity. “If that were true, then the second infection would be perhaps surprising, but not shocking.”

A series of tests could help ascertain what happened to Rahm, including assessing antibody levels. That data would speak to the strength of Rahm’s immune response. It’s likely the golfer had a robust one given his age. If the immune response were weak, that could suggest some sort of immune-response deficiency, said Dr. Perlman, the University of Iowa virologist who’s also trained as a physician. 

Research suggests that people who are immunocompromised don’t respond as well to vaccines. Some health agencies have already recommended booster shots for this population, and the U.S. Centers for Disease Control and Prevention recently signaled its support for this use case. 

Dr. Perlman said he would also look to measure how much virus Rahm was shedding. The lower the amount, the less likely contagion is, he and others said. 

One caveat: sometimes people test positive for the virus long after they’ve recovered. That’s because tests are designed to detect viral genes, not live virus particles. So it’s possible the tests are picking up just fragments that can’t replicate and infect.

In other words, Rahm might not be contagious at all, potentially trapping him in a recurring nightmare. 

“This has implications for him professionally. This is a big deal,” said Krutika Kuppalli, assistant professor of infectious diseases at the Medical University of South Carolina. “If I were on the committee coming up with the [Olympic] policies, this would be one of the scenarios I’d think about.”

Ascertaining that would require lab technicians to culture live virus from Rahm’s sample, she said. If the virus didn’t grow, that would suggest the sample didn’t contain contagious particles.  

Jon Rahm found out about a positive test after the third round of the Memorial Tournament in June.

Jon Rahm found out about a positive test after the third round of the Memorial Tournament in June.

Photo: Shelley Lipton/Zuma Press

It’s a situation the biggest, richest sports leagues in the world seem destined to find themselves in. Sports in 2020 designed extensive and often draconian protocols to limit the virus’s transmission among players, coaches and staff. Leagues like the NFL, NBA and MLB in 2021 are increasingly designing their policies around vaccines while dramatically scaling back the older protocols. 

And now leagues will have to decide if they need to treat athletes who are re-infected, or others who have breakthrough infections, differently than an unvaccinated person who contracts Covid-19. It could boil down to understanding just how infectious people like Rahm are. 

But that also adds complexity.  “That’s why the rules are so straight-laced,” said Dr. Perlman. “There are so many caveats if you start becoming nuanced. It’s hard to know where you’ll end up.”

Write to Daniela Hernandez at daniela.hernandez@wsj.com and Andrew Beaton at andrew.beaton@wsj.com