More Evidence of Protective Immunity After COVID-19

Most COVID-19 patients develop neutralizing antibodies against SARS-CoV-2. Whether such antibodies make them immune to SARS-CoV-2 infection is unclear, especially after reports of patients who appeared to have contracted SARS-CoV-2 twice within a few weeks. Now, an observational study of a COVID-19 outbreak on a fishing vessel with 122 crew members (113 men and 9 women) sheds some light on this issue.

Investigators took nasopharyngeal swabs for reverse transcription polymerase chain reaction (RT-PCR) testing and blood for serological testing from 120 of the 122 crew members ≤2 days before departure. All of these individuals had negative RT-PCR results, but the SARS-CoV-2 antibody screening test was reactive in six of them. On day 18 of the trip, the ship had to return to shore after one crew member developed RT-PCR–confirmed SARS-CoV-2 infection and required hospitalization. Subsequently, all other crew members underwent RT-PCR testing, and 114 provided samples for serology.

During a median follow-up of 32.5 days, 101 crew members tested positive for SARS-CoV-2 by RT-PCR. Most of these crew members also seroconverted, as did 3 of the 21 with negative RT-PCR results, resulting in an 85% attack rate on board. Sequencing of 39 samples confirmed that the outbreak stemmed from a single viral clade. Of the six crew members with predeparture reactive antibody screening tests, only three actually showed neutralizing antibodies with typical postinfection titers. None of these individuals tested positive for SARS-CoV-2, while the other three developed RT-PCR–confirmed infection, yielding an 88% infection rate in individuals without neutralizing antibodies (P<0.002).