Age-Stratified Seroprevalence of SARS-CoV-2 Antibodies before and during the Vaccination Era, Japan, February 2020-March 2022

Abstract

Japan has reported a relatively small number of COVID-19 cases. Because not all infected persons receive diagnostic tests for COVID-19, the reported number must be lower than the actual number of infections. We assessed SARS-CoV-2 seroprevalence by analyzing >60,000 samples collected in Japan (Tokyo Metropolitan Area and Hokkaido Prefecture) during February 2020-March 2022. The results showed that ≈3.8% of the population had become seropositive by January 2021. The seroprevalence increased with the administration of vaccinations; however, among the elderly, seroprevalence was not as high as the vaccination rate. Among children, who were not eligible for vaccination, infection was spread during the epidemic waves caused by the SARS-CoV-2 Delta and Omicron variants. Nevertheless, seroprevalence for unvaccinated children <5 years of age was as low as 10% as of March 2022. Our study underscores the low incidence of SARS-CoV-2 infection in Japan and the effects of vaccination on immunity at the population level.

Keywords: COVID-19; Japan; SARS-CoV-2; antibody; coronavirus disease; epidemiology; incidence; respiratory infections; seroprevalence; severe acute respiratory syndrome coronavirus 2; vaccine; vaccine-preventable diseases; viruses; zoonoses.

Figure 1

Epidemic curve of COVID-19 in Japan, January 2020–March 2022. The daily numbers of reported COVID-19 cases per100 persons in all of Japan, the Tokyo Metropolitan Area, and Hokkaido Prefecture are shown. The numbers indicate the 6 epidemic waves. The fourth, fifth, and sixth waves were driven by the Alpha, Delta, and Omicron variants of SARS-CoV-2, respectively.

Figure 2

Seroprevalence of SARS-CoV-2 in the Tokyo Metropolitan Area, Japan, February 2020–March 2022. A) Rates for the total population of the Tokyo Metropolitan Area; B–F) rates by 20-year age groups. The cumulative number of reported COVID-19 cases and the cumulative number for the first, second, and third vaccine administrations per population are also shown. Error bars indicate 95% CIs. Detailed age-stratified data are shown in Appendix Figure 2).

Figure 3

Ratios of SARS-CoV-2 seroprevalence to cumulative incidence by month in the Tokyo Metropolitan Area, Japan, May 2020–March 2021. Ratios for each month were calculated in comparison to the cumulative incidence of reported COVID-19 from January 2020 to that month. The ratio corresponds to the actual number of infected persons per reported case-patient. Error bars indicate 95% CIs. Data are blank for months when no samples were positive for SARS-CoV-2.