Fig. 1: Schematic diagram of this study and serum anti-N antibody level dynamics.

a Overview of the seroepidemiological survey conducted in Japan for this study. Serum samples and self-administered questionnaires were collected in two rounds conducted in December 2022 and February 2023. Sampling and data collection were carried out over several days at 15 sites (three municipalities in each of five prefectures). The periods of the two surveys correspond to the gray areas in the graph of COVID-19 case counts in Japan (https://www.mhlw.go.jp/stf/covid-19/open-data.html). The December 2022 survey was conducted from November 26, 2022, to December 27, 2022, while the February 2023 survey was conducted from February 3, 2023, to March 4, 2023. b Flow diagram of the study design and inclusion of participants. See Method for details. c A model of the dynamics of anti-N antibody levels in diagnosed primary infection (n = 1218) and re-infection (n = 26). Each data point (dots), along with the median (lines) and 95% credible intervals (CredIs) (ribbon) for the mean anti-N antibody levels, is shown. The dark gray dotted line indicates the cutoff index (1.0 COI) according to the manufacturer’s manual, and the light gray dotted line represents the detection limit (0.1 COI). d Estimation of the fold increase in anti-N antibody levels induced by re-infection. The fold increase was calculated from the ratio of estimated anti-N antibody levels on the days post-primary diagnosis (infection) at re-infection to the titers at corresponding days post-re-infection. The white dotted and solid lines show a 2× and 4× fold increase, respectively, in the anti-N antibody level. The black line shows the median observation period (63 days).