HIV-1 incidence determined retrospectively among drug users in Bangkok, Thailand

To measure trends in the incidence of HIV-1 infection among drug users in treatment at Thailand's largest drug detoxification unit.

A retrospective cohort was established using computed, existing HIV-1 test results of 26,396 inpatients and outpatients admitted for 47,907 drug detoxification treatment courses from August 1987 to August 1992.

Matching of patient record numbers showed that 10,050 (38.1%) patients had been admitted two or more times during the period. From these, we selected a cohort of 7807 initially HIV-negative patients. Subsequent seroconversions among them were assumed to have occurred with uniform probability throughout the interval between the last HIV-negative and the first HIV-positive tests.

There were 2311 (29.6%) seroconversions in the cohort. HIV-1 incidence among the 5974 (76.5%) who were injecting drug users (IDU) escalated from 20 new infections per 100 person-years (PY) of observation in 1987 to a peak of 57 per 100 PY in 1988, then gradually declining to a stable rate of about 11 per 100 PY during 1991 and 1992. Non-IDU (smokers, inhalers) constituted 683 (8.8%) of the cohort patients, and had HIV-1 incidence rates varying from 0.2 to five per 100 PY. 'Mixed' drug users, defined as individuals reporting different routes of drug administration on different admissions, composed 1150 (14.7%) of cohort patients and had an HIV-1 incidence rate between that of IDU and non-IDU. Prevalence of HIV-1 seropositivity among all IDU increased rapidly, from about 1% in early 1988 to a peak of about 40% by early 1989, and has remained stable through 1992.

Prevention efforts must continue for IDU, since recent annual HIV-1 incidence remains high at > 10 per 100 PY. Such a high rate suggests that this group should be considered for HIV-1 vaccine efficacy trials. Stable HIV-1 prevalence can mask substantial incidence in a population with high turnover.