Japan’s HTLV-1 carrier population projected to fall 78% by 2060 as transmission shifts to ages 16–29

Japan’s nationwide program to prevent mother-to-child transmission of human T-cell leukemia virus type 1 (HTLV-1) is projected to cut the country’s carrier population to roughly 143,546 people by 2060, an approximately 78% reduction compared with the 2020 estimate, according to a modeling study published in BMC Infectious Diseases. The projection, which extends 50 years beyond the program’s 2011 launch, is presented as the first long-term quantitative forecast of HTLV-1 transmission dynamics in Japan built on age-specific transmission coefficients derived from nationwide seroconversion data.

Japan implemented its nationwide mother-to-child transmission (MTCT) prevention program in 2011, but the long-term epidemiological consequences of that strategy had remained unclear. The research team addressed this gap by developing a projection model that separately quantified newly generated HTLV-1 infections and the surviving carrier population, using nationwide epidemiological data as its foundation.

The model estimated new infections through two distinct pathways. Infections arising through mother-to-child transmission were projected alongside horizontal transmission, which the researchers calculated 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.

The results reveal a sharply asymmetric future for the two transmission routes. Between 2011 and 2060, only 1,611 new MTCT infections are projected to occur. By contrast, 102,528 new infections are expected to arise through horizontal transmission between 2021 and 2060. The projected incidence of MTCT among children born after the nationwide prevention program was implemented is just 0.005%, which the authors describe as indicating substantial success of the current prevention strategy.

That success, however, comes with an epidemiological trade-off. With new infections at birth projected at very low levels, horizontal transmission is expected to become the dominant route of newly generated HTLV-1 infection in Japan. In the model’s projections, the highest horizontal transmission potential was observed among adolescents and young adults aged 16 to 29 years, an age band that accounts for the largest share of ongoing viral spread in the forecast.

The projected decline in carrier numbers emerges from the model’s two components: the small projected inflow of newly generated infections and the application of age-specific survival probabilities to the carrier cohorts already carrying the virus. By separating these components, the researchers were able to project both the future flow of new infections and the surviving carrier total through mid-century.

The study’s authors conclude that 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. This finding highlights, they write, the need to expand future national HTLV-1 control strategies beyond MTCT prevention to include comprehensive interventions targeting horizontal transmission, particularly among the age groups where transmission potential is highest.

As with any projection model, the estimates depend on the assumptions embedded in the underlying data, including the age-specific transmission coefficients and survival probabilities drawn from nationwide observations. The projections describe expected trends under the current prevention framework rather than measured future outcomes, and the published version is an early-release, peer-reviewed accepted article that remains subject to further edits before the final Version of Record. Even so, the analysis offers public health planners in Japan a quantitative horizon: a shrinking but persistent carrier population, very low projected numbers of infant infections, and a growing share of newly generated transmission concentrated among adolescents and young adults aged 16 to 29 through 2060.

Subject of Research: Long-term mathematical projection of HTLV-1 transmission dynamics and carrier numbers in Japan through 2060

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

Article References: Nagata, K., Sagara, Y., Nakamura, H., Kuramitsu, M., Tezuka, K., Hasegawa, Y., Komatsu, N., Eishi, C., Nishi, M., Nagata, A., Miura, S., & Miura, K. (2026). Long-term projection of HTLV-1 transmission dynamics in Japan through 2060. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14697-2

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14697-2

Keywords: HTLV-1, Japan, mathematical modeling, mother-to-child transmission, horizontal transmission, epidemiology, projection model, BMC Infectious Diseases, Long-term, projection, transmission, dynamics