Estimation of the acquired immunodeficiency syndrome incubation period in intravenous drug users: a comparison with male homosexuals.

Estimates of the risk of developing acquired immunodeficiency syndrome (AIDS) have been limited to studies involving homosexual men, transfusion recipients, and hemophiliac subjects. Little is known, however, about the natural history of the human immunodeficiency virus infection in intravenous drug users. An Italian multicenter cohort study of 420 individuals who seroconverted between 1982 and 1990 provided the opportunity to study the incubation period to AIDS in this group. A three-state Markov model was fitted to the data, and estimates of the risk of developing AIDS were obtained for intravenous drug users, male homosexuals, and three age groups. The risk of developing AIDS increased significantly with age. The estimated annual risk of developing AIDS for individuals between 25 and 34 years of age and over 35 years was 2.4 and 3.8 times higher, respectively, than that observed for individuals between 16 and 24 years of age. The mode of transmission did appear to have a small but not statistically significant influence on the time interval between the diagnosis of AIDS-related conditions and AIDS.