MODELING INCIDENCE RATE RATIO AND RATE DIFFERENCE - ADDITIVITY OR MULTIPLICATIVITY OF HUMAN-IMMUNODEFICIENCY-VIRUS PARENTERAL AND SEXUAL TRANSMISSION AMONG INTRAVENOUS-DRUG-USERS

The analysis concerns data from the Northern Italian Seronegative Drug Addicts Study, a multicenter longitudinal study about the incidence o f human immunodeficiency virus infection in intravenous drug users fro m Milan and other areas of northern Italy between 1987 and 1991. Diffe rent measures of parenteral and heterosexual exposure effects were est imated by fitting multiplicative models for rate ratio and additive mo dels for both rate ratio and rate difference into a Poisson regression model for grouped cohort data. In areas of high human immunodeficienc y virus prevalence among intravenous drug users, the adjusted rate rat io under a multiplicative structure was 6.2 (95% likelihood-based conf idence interval (LCI) 2.9-14.4) for parenteral and 2.9 (95% LCI 1.3-6. 1) for sexual transmission. Under the additive model, the rate ratio w as 7.8 (95% LCI 3.4-20.2) for parenteral and 9.2 (95% LCI 2.2-29.7) fo r sexual transmission, and the rate difference per 100 person-years wa s 9.8 (95% LCI 5.3-15.6) for parenteral and 10.5 (95% LCI 1.8-24.2) fo r sexual transmission (controlled for each other). Because of the smal l sample size, a clear discrimination between models could not be reac hed. However, in spite of the greater risk associated with parenteral transmission under a multiplicative model, the additive model suggests that the relative impact of measures aimed at inducing condom use is similar to that which would be obtained by measures aimed at stopping syringe sharing.