Dynamic change of COVID-19 seroprevalence among asymptomatic office workers in Tokyo from May through Dec 2020 during the second waves of COVID-19.

Abstract Background COVID-19 deaths per capita in Japan have been low compared to Western Countries despite the absence of the lockdown. It is still unclear either the less spreading of the virus or, the less progression to the severe illness. Therefore, it is crucial to determine the seropositivity rates (SPR) in the general population. We determined SPR in healthy office workers in Tokyo with validation of results by comparing two test results from the same individual a month apart.Method Healthy office workers for a Japanese company in Tokyo were determined seropositivity against COVID19 weekly from May 26 to August 25, 2020, by a rapid COVID19 IgM/IgG test kit using fingertip blood. SPR was calculated by pooled data from each two-week window. For each participant, tests were offered twice, separated by a month, to provide self-reference to validate the results. The seropositive individuals were offered a follow-up test on Dec 8th or 15th, 2020, when the third wave loomed in Tokyo, to determine the persistence of the seropositivity.Results 615 volunteers (mean + SD 40.8 + 10.0; 45.7 % female) from 11 discrete locations received at least one test. 350 individuals completed two tests a month apart. The comparison showed that 93.2 % showed reasonable status changes and no instances with physiologically unexpected changes (e.g., IgG (+) turns IgM(+) in a month). Seroprevalence increased from 5.8 % to 46.8 %, paralleling the rise of the cases in Tokyo. Among 152 seropositive participants, 74 participants underwent the follow-up test in Dec 2020. 40.5 % turned seronegative in four months (mean ± S.D. 120 ± 17 days).Conclusion COVID-19 infection may have spread widely in Tokyo despite the low fatality. Given the temporal correlation between the rise in seropositivity and the peak out of COVID-19 cases without a lockdown, temporary herd immunity might be achieved. However, the relatively short-lived antibodies among the asymptomatic individuals may explain why the herd immunity strategy has not succeeded, and COVID-19 resurgence happens every several months. Moreover, sequential testing for serological response against COVID-19 is useful for understanding the dynamics of COVID-19 infection at the population level.