Using linked national registry data and the ECDC HIV modelling tool to estimate HIV incidence and the proportion diagnosed in Norway up to 2023

Estimates of HIV incidence and the size of the undiagnosed population are important to monitor epidemic trends, assess progress towards regional and global goals, and inform ongoing prevention and control measures. Previous estimates from Norway date back to 2018 and did not account for pre-migration infection among migrants. This is essential when modelling the HIV epidemic in countries like Norway, where over 80% of new HIV diagnoses are among migrants who reportedly acquired HIV before migration. I estimated the incidence of new HIV infections, number undiagnosed and proportion diagnosed overall and in key groups at risk of HIV acquisition up until the end of 2023 in Norway. I used linked national registry data and the ECDC HIV modelling tool, a back-calculation compartmental model that uses routine surveillance data on HIV diagnoses. The modelling tool estimates pre-migration infections by comparing the known date of migration to the estimated date of infection. I generated estimates overall and for four subgroups: Norwegian-born men who have sex with men (MSM), Norwegian-born heterosexual transmission, migrant MSM and migrant heterosexual transmission. The model input data included 7,185 new HIV diagnoses up to the end of 2023, of which 4,266 (59%) were migrants. The model estimated that 86% (95% confidence interval (CI): 83% – 88%) of new infections among migrants were pre-migration infections. Excluding these, there were 13 (95% CI: 11 – 50) new HIV infections in Norway in 2023. There were 234 (95% CI: 207 – 336) undiagnosed PLHIV and the proportion diagnosed was 96.5% (95% CI: 95.1% – 96.9%). The diagnosed fraction was under 95% among Norwegian-born heterosexual transmission (92.6%; 95% CI: 87.1% – 94.5%) and migrant MSM (92.2%; 95% CI: 90.8% – 93.8%). Overall, Norway has achieved the global target of diagnosing 95% of people living with HIV. The incidence of new HIV infections is low. Existing measures should be maintained and strengthened to further reduce the undiagnosed population and prevent transmission. The lower proportion diagnosed among Norwegian-born persons infected through heterosexual transmission and migrant MSM indicates where larger relative improvements may be made.