National seroepidemiological study of COVID‐19 after the initial rollout of vaccines: Before and at the peak of the Omicron‐dominant period in Japan

Abstract

Background Based on routine surveillance data, Japan has been affected much less by COVID‐19 compared with other countries. To validate this, we aimed to estimate SARS‐CoV‐2 seroprevalence and examine sociodemographic factors associated with cumulative infection in Japan. Methods A population‐based serial cross‐sectional seroepidemiological investigation was conducted in five prefectures in December 2021 (pre‐Omicron) and February–March 2022 (Omicron [BA.1/BA.2]‐peak). Anti‐nucleocapsid and anti‐spike antibodies were measured to detect infection‐induced and vaccine/infection‐induced antibodies, respectively. Logistic regression was used to identify associations between various factors and past infection. Results Among 16 296 participants (median age: 53 [43–64] years), overall prevalence of infection‐induced antibodies was 2.2% (95% CI: 1.9–2.5%) in December 2021 and 3.5% (95% CI: 3.1–3.9%) in February–March 2022. Factors associated with past infection included those residing in urban prefectures (Tokyo: aOR 3.37 [95% CI: 2.31–4.91], Osaka: aOR 3.23 [95% CI: 2.17–4.80]), older age groups (60s: aOR 0.47 [95% CI 0.29–0.74], 70s: aOR 0.41 [95% CI 0.24–0.70]), being vaccinated (twice: aOR 0.41 [95% CI: 0.28–0.61], three times: aOR 0.21 [95% CI: 0.12–0.36]), individuals engaged in occupations such as long‐term care workers (aOR: 3.13 [95% CI: 1.47–6.66]), childcare workers (aOR: 3.63 [95% CI: 1.60–8.24]), food service workers (aOR: 3.09 [95% CI: 1.50–6.35]), and history of household contact (aOR: 26.4 [95% CI: 20.0–34.8]) or non‐household contact (aOR: 5.21 [95% CI:3.80–7.14]) in February–March 2022. Almost all vaccinated individuals (15 670/15 681) acquired binding antibodies with higher titers among booster dose recipients. Conclusions Before Omicron, the cumulative burden was >10 times lower in Japan (2.2%) compared with the US (33%), the UK (25%), or global estimates (45%), but most developed antibodies owing to vaccination.

Seroprevalence of severe acute respiratory syndrome coronavirus 2 N antibodies between December 2021 and march 2023 in Japan

“…However, the seroprevalence progressively increased to approximately 23% after the emergence of the omicron variants. The seroprevalence of antibodies against the N protein in Japan was reported to be 3.5% in February to March 2022 [1] and cumulative confirmed cases as of May 2023 exceeded 33.8 million (28.2% of the Japanese population). Unlike other countries where seroprevalence rates exceeded 50% [3,4,[6][7][8][9][10], most of the Japanese population had not been infected at that time, suggesting the possibility of future SARS-CoV-2 epidemics in Japan.…”

“…As of May 2023, 760 million cases including 6.9 million deaths have been reported to the World Health Organization (https://covid19.who.int/). The threat of SARS-CoV-2 is subsiding as most people have acquired sufficient antibodies through vaccination and/or infection to prevent severe COVID-19; more than 80% of the populations of major industrialized countries have antibodies against the S protein induced by vaccination and/or infection [1][2][3][4][5]. In contrast, the seroprevalence of antibodies against the N protein elicited by SARS-CoV-2 infection ranged from 44.4% to 80.2% in the United States, England, Spain, Slovenia, Switzerland, and Thailand between March and November 2022 after the emergence of the omicron variants [3,4,[6][7][8][9][10].…”

“…In March 2022, cumulative confirmed cases in Japan were approximately 6.6 million, representing 5.5% of the Japanese population, and the seroprevalence of antibodies against the N protein was 3.5% [ 1 ]. This rate was significantly lower than that of the aforementioned countries.…”

Estimating serum cross-neutralizing responses to SARS-CoV-2 Omicron sub-lineages elicited by pre-Omicron or Omicron breakthrough infection with exposure interval compensation modeling

“…Fourth, this study did not include any individuals with a second booster dose of vaccine or breakthrough infection after the first booster dose of vaccine. Finally, our investigation did not evaluate the actual risk of reinfection by SARS-CoV-2 in individuals with a history of breakthrough infection, although there is evidence that NTs are correlated with protection against ancestral strains and different variants (30)(31)(32).…”

Infection-Induced SARS-CoV-2 Seroprevalence among Blood Donors, Japan, 2022

“…During June 2020–February 2022, four large-scale serologic surveys were conducted in 5 prefectures of Japan (Miyagi, Tokyo, Aichi, Osaka, and Fukuoka) ( 6 , 7 ); a comprehensive survey covering all 47 prefectures has not yet been conducted. Seroprevalence during SARS-CoV-2 Omicron variant predominance in February 2022 was 3.5% for the 5 prefectures ( 6 ).…”