Changes in antibody titers over time after infection in Bizen City, Japan.

The proportion of people who have anti-nucleocapsid (N) protein antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been reported as a surrogate for the proportion of previously infected individuals. This seroepidemiologic survey, conducted from 6 November 2023 to 19 January 2024, aimed to measure the levels of anti-N protein antibodies produced in the body by SARS-CoV-2 among residents over 18 years of age in Bizen City, Okayama, Japan. We evaluated the prevalence of SARS-CoV-2 antibodies in the target population at the time of the investigation and assessed changes over time among participants who had previously been infected. Of the 444 participants, a total of 235 (52.9%) were infected within one year. The seroprevalence of participants testing positive remained above 80% until four months post-infection, after which the number of people who remained positive declined rapidly. Six months after infection, around 10% of participants tested positive for anti-N protein antibodies, and this seroprevalence remained the same for up to one year post-infection. We demonstrated that the anti-N protein antibodies test is useful for identifying recently infected individuals for several months post-infection. We showed that it becomes increasingly difficult to detect past infections as time elapses since infection.