Progress towards HIV transmission elimination targets: model-based estimation of incidence and the extent of undiagnosed infection, Scotland, 1981 to 2022.

BACKGROUNDThe global goal to end the AIDS epidemic cannot be achieved without estimates of incidence and undiagnosed infection.AIMWe aimed to estimate the timing of HIV transmission events and the number of people unaware of their diagnosis in Scotland, by mode of acquisition and migrant status.METHODSSurveillance data from Scotland's national HIV diagnosis database (1981-2022) linked to death and migration data was entered into the HIV Platform tool of the European Centre for Disease Prevention and Control, to back-calculate HIV incidence while imputing missing data and adjusting for reporting delay.RESULTSWe estimated 8,235 HIV transmission events between 1980 and 2022 among people living in Scotland, with an 80% reduction from 2010 to 2021 (258 to 52 events). Excluding people diagnosed outside Scotland, we estimated 4,854 (95% confidence interval (CI): 4,637-5,080) people living with HIV at the end of 2021, of whom 8.2% (396/4,854) were undiagnosed. Stratified estimates of this proportion were 6.9% for gay, bisexual and other men who have sex with men, 7.0% for people who inject drugs, 12.6% and 12.4% for heterosexuals born and not-born in the United Kingdom (UK), respectively. Including people first diagnosed with HIV outside Scotland, the overall proportion undiagnosed was 6.1% (396/6,444).CONCLUSIONScotland is on track to meet the United Nations' diagnosis target of 95% by 2025, with the World Health Organization target of a 75% incidence reduction met since 2010. To reduce further transmission, expanded HIV testing and prevention services are necessary to better reach at-risk heterosexual individuals.