Understanding the long-term course of HIV epidemics

Field: Medicine (Health Sciences)

In this issue, Van Ameijden and Coutinho [1] present important information on the course of the HIV epidemic amongst injecting drug users (IDU) in Amsterdam over the last 10 years. HIV prevalence reached a high level in Amsterdam (approximately 35%) in the early 1980s, prior to awareness of the threat of AIDS. Amsterdam organisations then implemented a full set of HIV prevention activities, including syringe exchange, methadone treatment and IDU organizations. Risk behavior was sharply reduced, although not eliminated. HIV incidence declined sharply from an estimated 8 per 100 person years at risk (PYAR) prior to implementation of the prevention activities, but has apparently stabilized at 3–4 per 100 PYAR. These results are not surprising, since the same research group has previously reported an incidence of 4 per 100 PYAR amongst IDU using their methadone program, those using their syringe exchange, and those not using either of these services [2]. This incidence rate cannot be considered acceptable from a public health perspective, and new prevention programs, particularly programs to reduce initiation into illicit drug injection, are clearly justified for Amsterdam. Understanding the long-term dynamics of high seroprevalence epidemics is one of the most important questions in HIV epidemiology (along with the question of how to prevent such epidemics in the first place). Once HIV seroprevalence reaches moderate-to-high levels, with a large number of potential transmitters, even modest levels of risk behavior typically lead to moderate-to-high incidence rates [3]. The pattern of HIV prevalence reaching a high level (≥ 30%), followed by risk reduction without risk elimination and a reduced but still unacceptably high incidence rate may be typical of high seroprevalence HIV epidemics among IDU, and perhaps also of high seroprevalence epidemics among men who have sex with men [4]. There also may be important variations on this pattern. Seroprevalence reached a high level amongst IDU in Bangkok, prevention programming was implementation, and risk reduction occurred [5]. The current incidence rate, however, appears to have stabilized at approximately 10 per 100 PYAR [6] substantially higher than the rate in Amsterdam. There may also be high seroprevalence cities where the incidence rate is lower than that reported for Amsterdam. For example, in New York City, incidence among methadone patients is less than 1 per 100 PYAR [7], and incidence among syringe exchange participants is 1.5 per 100 PYAR, both below the comparable figures for Amsterdam. Current incidence in the New York City HIV Vaccine Network IDU cohort study (using methods similar to the Amsterdam cohort study) is approximately 1 per 100 PYAR (M. Marmor, personal communication, 1997). Detailed cross-city comparisons are needed to identify the reasons for these differences in long-term HIV incidence. Understanding differences in long-term incidence rates in cities with high seroprevalence will require measuring more than the standard variable of the percentage of IDU who report injecting with shared equipment in the 6 months prior to interview. Van Ameijden and Coutinho [1] report that this risk behavior has stabilized at approximately 30% in the intake interviews for the Amsterdam cohort. This value is not significantly different from 43% observed in cross-sectional studies of IDU in Bangkok [8], 27% currently observed amongst IDU in New York City (unpublished data), and 39% amongst IDU in Vancouver, Canada, where recent incidence is estimated at approximately 18 per 100 PYAR [9]. The modest differences in the percentages of IDU who have receptively shared injecting equipment in the last 6 months would not appear to explain the rather large differences in estimated HIV incidence. Measures that include not only the presence/absence of injecting risk behavior but also the frequency of risk behavior and mixing patterns among risk ‘partners’ will probably be necessary to understand both continuing HIV transmission dynamics and the limits of risk reduction among populations of IDU. A measure of infectivity amongst HIV-seropositive patients (perhaps using viral load) would also be useful. The Amsterdam studies have made very important contributions by showing substantial reductions in risk behavior and in HIV incidence in association with a wide range of HIV prevention programs for IDU. The most recent report shows that this set of prevention activities may not be sufficient to bring a high sero-prevalence epidemic under control. Additional interventions, including programs to reduce initiation into drug injecting, are needed in Amsterdam and in many other cities. We also need better measurement and understanding of the dynamics of continuing risk behavior and continuing HIV transmission in populations with high HIV seroprevalence.