Rubella immunization policy in Japan: Immunity gaps arising from program differences and inconsistent cutoff standards

Rubella elimination remains a global health priority due to the risk of congenital rubella syndrome (CRS) following maternal infection during early pregnancy. Japan's immunization policy began in 1977 with female-only vaccination, reflecting a narrow focus on CRS prevention, and gradually expanded to universal childhood immunization. However, men born between 1962 and 1978-the so-called "lost generation" - were left with few vaccination opportunities, leading to major outbreaks in 2012-2013 and 2018-2019. In response, the government introduced the Phase 5 catch-up program in 2019, while women planning pregnancy and their partners were addressed through the CRS prevention program, which applies stricter serological cutoffs. Our single-institution data illustrate how program design and inconsistent cutoff standards generate apparent immunity gaps: the Phase 5 cohort showed seropositivity rates consistent with national surveys, whereas the CRS cohort revealed unexpectedly low immunity, particularly among young women. Applying a unified cutoff reduced these differences, underscoring the influence of threshold setting and participant selection. Japan's experience highlights the need to harmonize standards, strengthen interventions among young women, and sustain adult surveillance. More broadly, it offers international lessons on how policy design and inconsistent criteria can shape the assessment of immunity and the success of elimination strategies.