A fishing vessel that left Seattle in May returned with an unexpected catch: the first direct evidence in humans that antibodies to the coronavirus can thwart infection.

More than a hundred crew members aboard the American Dynasty were stricken by the infection over 18 days at sea. But three sailors who initially carried antibodies remained virus-free, according to a new report.

Although the study is small, it addresses one of the most important questions in the pandemic: whether the immune response to one bout with the virus protects against reinfection.

“Knowing the answer to this question is critical for vaccine design and epidemiology,” tweeted Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle and one of the study’s authors.

The study was posted online last week and has not yet been published in a peer-reviewed journal. Still, the finding set off optimistic chatter among scientists, who have been relying on monkey studies for evidence of antibodies’ potency.

“I thought it was very exciting — good enough news that I was telling my family about it,” said Michal Tal, an immunologist at Stanford University who was not involved in the work.

Several research teams have reported that an encounter with the virus triggers a robust immune response in most people, including in those who may have been only mildly ill. And the vaccine candidates now in trials also seem to elicit strong neutralizing antibodies, the kind that can block the virus.

But the amount of those antibodies needed to prevent the virus from returning is unclear. Scientists measure neutralizing antibodies in titers, an indication of their concentration in the blood.

The three sailors who remained protected from the virus had widely varying titers; two had only moderate quantities, a finding the researchers said was reassuring.

“People have been so worried about the titers, and the titers going down,” Dr. Alexander Greninger, a virologist at the University of Washington in Seattle, said.

The results indicate even moderate titers prevented reinfection in a situation in which exposure to the virus was high, he said: “These are attainable titers, right? Hopefully, it’ll be helpful to see, and makes make me very optimistic about the vaccines.”

The American Dynasty carried 113 men and nine women. All crew members had been tested for both virus and antibodies as part of a routine screening before setting sail. (The researchers did not have access to the results from two members.)

The trawler returned to shore after 18 days at sea when a crew member became ill enough to need hospitalization. The sailors were tested for the presence of virus and antibodies again and for up to 50 days after their return.

The three sailors confirmed to have neutralizing antibodies did not test positive for the virus during the course of the study; 103 of the remaining 117 became infected.

These numbers may be small, but they’re highly significant, Dr. Greninger said.

“A lot of people, when they see this are like, ‘Oh come on, it could be due to random chance,’” he said. In fact, the likelihood that the results are just chance is extremely low, he added.

Other experts agreed. “Just looking at the numbers, it becomes clear that it’s unlikely that all of these three people were protected by chance,” said Florian Krammer, an immunologist at the Icahn School of Medicine at Mount Sinai in New York.

Dr. Krammer and his colleagues are tracking antibody levels in people who have recovered from the coronavirus once to see at what point they might be vulnerable to reinfection. The team began with people in New York, but the virus is circulating at such low levels in the city now that Dr. Krammer and his colleagues have had to expand the study to other locations.

Data from vaccine trials also will identify the antibody titers required to disarm the virus. But in the meantime, “this is the first evidence in humans,” Dr. Krammer said. “It made my weekend.”

The study raised other questions. Based on the Abbott Architect assay, six of the 120 people tested before the boat’s departure had antibodies to the virus indicating prior exposure.

But when the researchers reanalyzed those samples using more sophisticated tests, only three of the six were confirmed to have antibodies, suggesting that three test results were false positives.

The Abbott test is advertised as returning fewer than one false positive for every 100 samples. “That’s a little concerning that the Abbott may be a little less specific than we thought,” Dr. Tal said.

The researchers also looked at antibodies in the blood, as most teams do. But those levels may not be the same as those in the nose or in saliva, the two major entry points for infection, Dr. Tal added.

“We’re looking in the wrong place,” she said. “If we want to look at protection from reinfection, we need to be looking in the nose.”