Modelling based on Australian HIV notifications data suggests homosexual age mixing is primarily assortative

To understand age-specific sexual mixing patterns of men who have sex with men in Australia, for behavioral insight, and to determine which age groups are at greatest risk of acquiring HIV in the future.

Diagnoses data based on Australia's HIV surveillance system are analyzed with a mathematical sexual transmission model to estimate the age-mixing patterns of sexual partnerships amongst men who have sex with men. The degree of assortativity and the relative rate of partner acquisition across different age groups are estimated.

Although there are some differences between age groups, sexual mixing is primarily assortative for men of all ages, between completely random partner acquisition and fully assortative mixing, with greatest assortativity among middle-aged men. Men aged 25-34 years were shown to have the greatest level of sexual partner acquisition. There is little evidence to support disassortative mixing on average across the population.

Although there is a tendency for partners of a similar age to be chosen and the average age of HIV-infected people in Australia is relatively old, suggesting that the epidemic may eventually age out depending on death rates, there seems to be sufficient cross-age mixing and high sexual mixing in younger age groups to sustain the HIV epidemic over the medium to long term.