Age and risk of HIV infection in men who have sex with men

HIV has been documented in populations of homosexual and bisexual men for more than 15 years. Despite significant changes in sexual risk behavior within the gay community, HIV continues to spread at a high rate in populations of men who have sex with men (MSM). According to the US Centers for Disease Control and Prevention, MSM account for 54% of the cumulative classified adult/adolescent AIDS cases in the United States through 1996, and 48% of incident cases in 1996. An additional 7% of cumulative and 5% of 1996 incident cases were attributed to a dual exposure category of MSM and injecting drug use. These figures indicate that sexual exposure among MSM continues to be a major source of HIV infections in the United States. The question of whether, and to what extent, young age is a risk factor for HIV infection in MSM has generated considerable interest and debate. Some studies point to a 'young age risk effect', whereas others do not. The investigations as a whole vary considerably in research designs, outcome measures, operationalizations of the age variable, and other methodological dimensions. Thus, different conclusions may be reached when different subsets of studies are considered. The absence of a comprehensive published review of the literature has stifled progress in this area. This article provides such a review and examines whether unprotected anal intercourse is more common and risk of HIV seroconversion higher in younger than older MSM.