Increasing horizontal transmission of human T-cell leukemia virus type 1 in adolescents and young adults in Japan

•Novel HTLV-1 infection was estimated using data from seroconverted blood donors.•More than 150% increase of novel infection was observed in young donors of both sexes.•HTLV-1 prevention strategies in Japan must address ongoing horizontal transmission. Human T-cell leukemia virus type 1 (HTLV-1) is a causative agent of the life-threatening diseases, adult T-cell leukemia/lymphoma and HTLV-1-associated myelopathy. Following implementation of antenatal screening in Japan, novel transmission of HTLV-1 in adolescent and adult generations is expected to replace vertical transmission as the main route for transmission. To obtain the current status of HTLV-1 horizontal infection and to assess the fluctuation of transmission occurring among adolescents and adults in Japan. We followed-up 5,017,916 eligible repeat blood donors for 8 years from 2013 to 2021. We evaluated HTLV-1 transmission rate by age group (16–69 years-old), and calculated the total number of novel transmissions in Japan using demographic statistics published by the government of Japan. We identified 457 seroconverters (men, 203; women, 254) in a total of 19,244,604 person-years during the study period. The number of seroconversions per 100,000 person-years was 1.54 for men and 4.21 for women. An increase in the number of novel infections was observed in both sexes in adolescent and young adult generations despite the health bias of blood donors. We estimate that more than 2,800 new HTLV-1 infections occur annually in Japan. It is a serious concern that without immediate measures against new HTLV-1 infections, such as guideline formulation, an inclusion of HTLV as routine screening in sexual health services, an information campaign, and surveillance of the general population, novel HTLV-1 infection could continue to increase in Japan and be a source of global transmission.