MODELING THE SPREAD OF HIV IN SOCIAL NETWORKS OF INJECTING DRUG-USERS

Objective: To explore the risk of a future rise of HIV prevalence in p opulations of injecting drug users (IDU) with low HIV prevalence but c ontinuing risk behaviour, and to study the potential influence of prev ention measures on HIV incidence. Methods: A stochastic simulation mod el was used to describe a network of longterm buddy relationships in a population of IDU. HIV transmission took place when borrowing injecti ng equipment from an infected buddy or stranger. The probability of tr ansmission depended on the duration of infection. Individuals remained in the population on average for 10 years. Two surveys amongst IDU in The Netherlands containing information about risk behaviour were used to estimate model parameters. We investigated the effect of different prevention strategies. Results: Below a threshold sharing frequency t he epidemic never takes off; above the threshold there is a large stoc hastic variation in prevalence. After reduction of risk behaviour, HIV prevalence decreases very slowly. Reducing sharing with strangers is more effective than reducing the overall sharing frequency. Prevention focused on new IDU greatly reduces HIV incidence. Reduction of sharin g frequency in HIV-positive IDU has no significant influence on HIV in cidence at HIV testing rates of 10 and 50% per year, if infectivity is highest during primary infection. Conclusions: A stabilization of HIV prevalence does not exclude the possibility of a future rise. Predict ions about the future course of an epidemic are inherently uncertain. The effect of prevention programmes on HIV prevalence only becomes vis ible on a long time-scale. Social networks of IDU play an important ro le in transmission dynamics and success of prevention. (C) 1998 Lippin cott-Raven Publishers.