Enlarge /. Fishing vessels in Seattle.
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Evidence of protective immunity to the pandemic coronavirus has emerged after a recent COVID-19 outbreak that flooded the crew of a fishing vessel.
The SARS-CoV-2 coronavirus infected 104 of the 122 people on board, around 85 percent, during a short voyage. Looking through the data collected before and after the ship sailed, the researchers found that the 18 spared from infection were only the only three people on board who had strong, pre-existing immune responses to SARS-CoV-2 . In particular, the three seafarers were the only ones found to have SARS-CoV-2 neutralizing antibodies. These are proteins that circulate in the blood and completely sink the infectious virus.
The numbers are small and the finding is not final. In addition, the study was published on a pre-print server this month, meaning it has not been published or peer-reviewed by any scientific journal. However, experts say the study was done well and is important for netting data that suggests that strong, pre-existing immune responses due to a previous infection can actually protect someone from getting infected again with the virus.
"Although this is a small study, it offers a remarkable, real-world human experiment at a time when we lacked formal evidence that neutralizing antibodies actually offer protection from re-infection as predicted by animal models," Danny said Altmann, professor of immunology at Imperial College London, in a media statement.
For the study, researchers in Seattle, Washington, were able to test 120 members of the 122-strong crew before setting off to sea. They looked for active infections by looking for SARS-CoV-2 genetic material in the nose, and they looked for previous infections by looking for antibodies that develop towards the end of an infection. All 120 were negative for SARS-CoV-2 in their nose. However, six had antibodies to the pandemic virus.
Upon further examining these six with antibodies, only three had neutralizing antibodies, the researchers found. Although all antibodies indicate previous exposure to a virus, not all antibodies can neutralize viruses. Neutralizing antibodies are believed to be critical to protective immunity.
Researchers can't say for sure what happened to the three who had some antibodies but didn't neutralize any antibodies. Your best guess is that they simply had false positive test results and no real antibodies to SARS-CoV-2. However, it is also possible that they had decreasing antibody responses, possibly due to a distant infection at the beginning of the infection or a burgeoning antibody response in the early stages of infection. Regardless, all three were infected with SARS-CoV-2 after the ship set sail and the COVID-19 outbreak broke out.
When the ship returned with illness on board after about 16 days at sea, the researchers retested all crew members and followed them for up to 32 days. A total of 104 were infected, including one of the two crew members who had not originally tested them.
For the three with neutralizing antibodies, the first tests for SARS-CoV-2 genetic material in the nose were all negative. Two of the three tested negative at three different times – from the day you quit to 18 days later. The third crew member tested negative after three and ten days of disembarking. But in one fold of the data, that person had very weak positive tests seven days and 13 days after getting off the boat. The tests were not classified as positive due to preset criteria. However, it suggests that there was residual viral material in the person's nose – which has been seen in other studies.
Despite the limitations of the study and the quarks of the data, experts say this is well-crafted and helpful information. Jonathan Ball, Professor of Molecular Virology at the University of Nottingham, noted in a media statement that while the study was small and left unanswered, it "gives us important insights into the type of immunity that may protect against future infections".