American Dynasty, a commercial trawler, departed Seattle one day in May to fish for hake off the Washington coast. Before leaving, its 122 crew members were screened for the coronavirus using the highly accurate polymerase chain reaction (P.C.R.) method, and all the results came back negative. But because those tests are “good but not perfect” . . . SARS-CoV-2 found its way on board.
When a crew member fell seriously ill, the vessel returned to port, and almost everyone was tested for the virus again. The before-and-after results for 120 of the crew members were made available to researchers, who published a study about them in The Journal of Clinical Microbiology in August. In addition to the P.C.R. tests, the pre-voyage screenings also looked for neutralizing antibodies, or proteins generated by the immune system after exposure to the virus, which suggest that a person has been infected previously. Three crew members, it turned out, had those antibodies at the start of the trip. Of the 117 crew members who did not, 103 tested positive for the virus when they got back to shore — an 88 percent infection rate. If you were to randomly select three names from the ship’s manifest, the odds that all three would have tested negative are about 0.2 percent. Yet all three sailors with antibodies were spared.
The finding is believed to be the first direct evidence that antibodies protect against SARS-CoV-2 infection in humans, and it offers clues about what sort of concentrations might be needed to confer immunity. The amounts of antibodies present in the three sailors are “pretty attainable by the vaccines” in development, says Alex Greninger, a virologist at the University of Washington School of Medicine and the study’s senior author.