Objective: To assess the relative contribution of steady and casual partnerships to the incidence of HIV infection among homosexual men in Amsterdam, and to determine the effect of increasing sexually risky behaviours among both types of partners in the era of highly active antiretroviral therapy (HAART).
Methods: A mathematical model was developed for the spread of HIV infection among young homosexual men in Amsterdam after the introduction of HAART. The model describes the formation of both steady and casual partnerships. Behavioural parameters were estimated separately for steady and casual partners from the Amsterdam Cohort Study among young homosexual men. HIV incidence and the fraction of new infections attributed to casual contacts were calculated from the model, allowing for uncertainty in the increases in risky behaviour, the effect of HAART, and levels of HIV testing and HAART administration.
Results: Currently, 86% (range 74–90%) of new HIV infections occur within steady partnerships. A reduction of 75–99% in infectivity caused by HAART will be counterbalanced by increases of 50% (range 30–80%) in risky behaviour with steady partners, but not by increases of up to 100% with casual partners. If HIV testing is increased from 42 to 80% and HAART administration from 70 to 85%, then even an increase of 100% in risk-taking with steady partners will not outweigh the effect of HAART.
Conclusion: Most new HIV infections among homosexual men in Amsterdam occur within steady relationships. Prevention measures should address risky behaviour, specifically with steady partners, and the promotion of HIV testing.
From the aCluster of Infectious Diseases, Municipal Health Service, Amsterdam, the Netherlands; bDepartment of Social and Organisational Psychology, University of Utrecht, Utrecht, the Netherlands; cDepartment of Human Retrovirology, Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands; and dDepartment of Infectious Diseases Epidemiology, National Institute of Public Health and the Environment (RIVM), Bilthoven, the Netherlands.
Correspondence and requests for reprints to: Maria Xiridou, Amsterdam Municipal Health Service, Cluster of Infectious Diseases, Nieuwe Achtergracht 100, PO Box 2200, 1000 CE Amsterdam, the Netherlands. Tel: +31 20 5555229; fax: +31 20 5555533; e-mail: firstname.lastname@example.org
Received: 10 May 2002; revised: 24 October 2002; accepted: 11 November 2002.