Trend of HIV Incidence Rates Among Drug Users in an HIV Epicenter in Northern Thailand (1989-1997)

Objectives: to determine trends and associated risk factors of HIV incidence (1989-1997) in a drug abuse treatment clinic in northern Thailand where HIV is epidemic Design: retrospective cohort study Methods: Nine-years (1989-1997) of data (excluding names) from the logbook of drug abusers seeking treatments in Mae Chan Hospital in Chiangrai Thailand, were transcribed and doubleentered into separate computer files which were later validated against each other. For each patient, the dates of the first HIV negative, the last HIV negative, and the first HIV positive were determined. A retrospective cohort of drug users who were initially HIV-negative and treated for more than once was constructed. HIV seroconversion was assumed to follow a uniform distribution between the last negative and the first positive HIV tests. The incidence rates and their 95% confidence intervals (CI) were calculated. Results: Of the 378 repeat patients, 16 (4.2%) HIV seroconverted. This is equivalent to 5.11 per 100 person-years of observation (PYO) (95%C1=3.13-8.35). The incidence remained relatively stable over the study period while the prevalence was on the decline. The younger, Thai lowlanders, drug injectors had higher incidence rates than the older, ethnic minorities and drug smokers, respectively. Conclusion: Prevalence can give illusional results. It is necessary to know baseline HIV incidence to monitor and evaluate an HIV intervention program. J Epidemiol, 1999 ; 9 : 114-120