Long-term projection of HTLV-1 transmission dynamics in Japan through 2060

Background Japan implemented a nationwide mother-to-child transmission (MTCT) prevention program for human T-cell leukemia virus type 1 (HTLV-1) in 2011. However, the long-term impact of this strategy on HTLV-1 epidemiology remains unclear. This study aimed to evaluate the long-term impact of Japan’s nationwide MTCT prevention program by projecting HTLV-1 transmission dynamics and carrier numbers through 2060, 50 years after implementation. Methods Using nationwide epidemiological data, we developed a projection model that separately quantified newly generated HTLV-1 infections and the surviving carrier population. New infections through MTCT and horizontal transmission were estimated, with horizontal transmission projected using age-specific horizontal transmission coefficients (ATCs) derived from nationwide seroconversion data. Age-specific survival probabilities were then applied to the corresponding carrier cohorts, including newly infected individuals, to estimate the future HTLV-1 carrier population through 2060. Results The number of HTLV-1 carriers in Japan was projected to decrease to 143,546 by 2060, representing an approximately 78% reduction compared with the 2020 estimate. Between 2011 and 2060, 1,611 new MTCT infections were projected, whereas 102,528 new infections were expected to arise through horizontal transmission between 2021 and 2060. The projected incidence of MTCT among children born after implementation of the nationwide prevention program was only 0.005%, indicating substantial success of the current prevention strategy. In contrast, horizontal transmission was projected to become the dominant route of new HTLV-1 infection, with the highest transmission potential observed among adolescents and young adults aged 16–29 years. Conclusions This study provides the first long-term quantitative projection of HTLV-1 transmission dynamics through 2060 using an ATC-based model. Although Japan’s nationwide MTCT prevention program is expected to substantially reduce vertical transmission through 2060, the epidemiological burden of HTLV-1 infection is projected to shift from vertical to horizontal transmission. These findings highlight the need to expand future national HTLV-1 control strategies beyond MTCT prevention to include comprehensive interventions targeting horizontal transmission.