TEMPORAL TRENDS OF INCIDENT HUMAN-IMMUNODEFICIENCY-VIRUS INFECTION IN A COHORT OF INJECTING DRUG-USERS IN BALTIMORE, MD

Objective: To measure the temporal trends in the incidence of infection with human immunodeficiency virus in a cohort of injecting drug users in Baltimore, Md, between 1988 and 1992. Design: Study subjects were screened for antibodies to human immunodeficiency virus by enzyme-linked immunosorbent assay and confirmed with Western blot. They were followed up at 6-month intervals with repeated serologic screening and comprehensive interviews for human immunodeficiency virus risk factors. Setting: Special study clinic. Participants: A cohort of 2960 participants were recruited and screened between February 1988 and March 1989. Recruitment criteria included an age of 18 years or older, a history of illicit drug injection since 1978, and the absence of the acquired immunodeficiency syndrome; subjects were subsequently tested for human immunodeficiency virus antibodies. Most subjects (85%) were not receiving methadone treatment at baseline and were recruited by word of mouth. Main Outcome Measure: Human immunodeficiency virus seroconversion. Results: Of the 2247 seronegative participants at baseline, 1532 were followed up, and 188 (12.3%) had sero-converted by December 1992. The incidence of human immunodeficiency virus infection over time among users declined somewhat, especially among women; the overall incidence was 1.90 per 100 person-semesters, or 3.80% annually. The incidence, adjusted for gender, was higher in younger (<35 years) than older (greater than or equal to 35 years) subjects (relative incidence, 1.75; 95% confidence interval, 1.29 to 2.38) and in women compared with men, adjusted for age (relative incidence, 1.29; 95% confidence interval, 0.95 to 1.80). The relative incidence among active compared with inactive drug users adjusted for age and gender was 1.58 (95% confidence interval, 1.06 to 2.35). Conclusions: Although the incidence of human immunodeficiency virus infection in this cohort of injecting drug users in Baltimore declined somewhat during the 4 years of follow-up, especially among women, the persistent annual incidence of nearly 4% during 31/2 to 41/2 years of observation suggests the need for additional strategies for prevention of infection, especially among those who continue injecting drugs. Newer methods of prevention, such as provision of sterile injection equipment, an effective human immunodeficiency virus vaccine, and wider availability of effective treatment or prevention of drug addiction, are urgently needed in these high-risk populations.