Methadone treatment and risk of HIV infection in drug users without legal access to clean injection equipment.

The particular situation of the Swiss canton of Vaud (population 550,000) provides favourable observational conditions to assess the efficacy of a methadone treatment scheme in reducing the risk of human immunodeficiency virus (HIV) infection among drug users. On the one hand, the canton has a long tradition of methadone treatment dispensed by medical practitioners. On the other hand, no legal access to clean injection equipment was provided up to 1989. For the 754 drug addicts having entered at least one course of treatment at the end of 1988, HIV status was assessed through two surveys conducted at mid-1986 and at end 1988 among the private practitioners and in the screening centers, hospitals, medico-social institutions and prisons. The overall annual HIV seroconversion rate shifted only slightly from 13% in the first study period (1984 to mid-1986) to 11% in the second period (mid-1986 to end 1988). In both periods, patients no longer on treatment, mostly stable abstainers, were the less exposed to HIV infection with a relative risk of 0.65 (p < 0.05). For those still on treatment, the risk of infection was associated directly (p < 0.001) with the frequency of courses and inversely (p < 0.001) with the duration. Between patients with more than 18 months spent on treatment and those with less than 6 months, the relative risk gradient was 0.8 and 1.4 before mid-1986 and widened out to 0.3 and 2.1 later on. This is mainly due to an increasing HIV incidence among newcomers into treatment.(ABSTRACT TRUNCATED AT 250 WORDS)