Parenteral and sexual transmission of human immunodeficiency virus in intravenous drug users: a study of seroconversion. The Northern Italian Seronegative Drug Addicts (NISDA) Study.

To evaluate the role of parenteral and sexual transmission of human immunodeficiency virus, we studied seronegative intravenous drug users recruited from 25 drug dependence treatment centers in northern Italy. All attending intravenous drug users were asked for their consent and screened for antibodies to human immunodeficiency virus; those who were seronegative were enrolled, interviewed about their habits, and invited to follow-up visits. Between 1987 and 1989, 1,195 seronegative intravenous drug users were enrolled, 635 were followed up (mean duration, 11.9 months), and 35 seroconversions were observed. The incidence rate ratios were 3.3 (95% confidence interval (CI) 1.4-7.5) for subjects aged less than 20 years, 2.4 (95% CI 1.2-4.7) for less than 2 years of intravenous drug use, 2.2 (95% CI 0.9-5.5) for syringe sharing, and 1.0 for subjects with a sexual partner who had tested positive for human immunodeficiency virus. A case-control approach, using logistic regression and adjusting for sex, age, area, and prevalence, showed odds ratios of 13.2 (95% CI 3.1-56.8) for frequent syringe sharing and 4.0 (95% CI 1.5-10.4) for sexual contacts with seropositive partners; frequent use of condoms was associated with a reduction in risk that did not reach statistical significance. Parenteral transmission is the most important route of infection with the human immunodeficiency virus among intravenous drug users, and sexual transmission plays a relevant, additive role.