Joint Associations of Undetectable=Untransmittable Awareness and Social Proximity with Sexual Decision-Making With People Living With HIV: A Cross-Sectional Study in Japan.

While the message "Undetectable = Untransmittable" (U=U) is increasingly recognised in Japan, its impact-together with social proximity to people living with HIV (PLWH)-on sexual decision-making remains unclear. This study examined whether the combination of U=U awareness and social proximity is associated with attitudes toward sex with PLWH. An online survey was conducted from November 2022 to January 2023 in Japan primarily through a gay dating app. Participants were grouped into four categories based on U=U awareness and having a friend living with HIV. Poisson regression with robust error variance examined associations with reporting that learning a potential sexual partner is living with HIV would have "no" or "little" influence on the decision to have sex. A total of 4715 HIV-negative/unknown cisgender men and gender diverse individuals who have sex with men were included (median age: 36, IQR: 29-46); over two-thirds were aware of U=U. Of all participants, 26.5% had neither U=U awareness nor friends living with HIV, 52.5% had U=U awareness only, 3.1% had friends only, and 17.9% had both. Overall, 8.3% reported no or little influence of a potential sexual partner's HIV-positive status on their sexual decision-making. Compared with those with neither factor, having both was strongly associated with accepting attitudes (adjusted prevalence ratio: 4.53, 95% CI 3.24-6.32). Having a friend living with HIV alone was also significant, whereas U=U awareness only was not. Beyond promoting U=U, fostering opportunities for meaningful interaction with PLWH may help reduce stigma in sexual contexts.