Background: Drug resistance remains a concern in the HAART era. We undertook this study to evaluate the incidence and prevalence of HIV drug resistance after therapy in a populational setting, as influenced by the increasing success of available antiretroviral therapies.

Methods: The BC Centre for Excellence in HIV/AIDS Drug Treatment Program provides antiretrovirals and resistance testing free of charge to HIV-infected individuals in British Columbia. Longitudinal plasma viral load and genotypic resistance data were obtained from patients receiving ART from the drug treatment program (n = 7363 patients with 21,300 resistance tests from 5216 individuals) between July 1996 and September 2007. The incidence of successful plasma viral load suppression and of resistance to each of 4 ARV categories (lamivudine/emtricitabine [3TC/FTC], other NRTI, NNRTI, or protease inhibitor [PI]) was calculated for the population receiving therapy.

Results:  There has been a linear increase in the proportion of individuals with plasma viral load suppression over time (64.7% <50 copies/mL in 2000 to 85.8% in 2007; r² = 0.98, p <0.001). Concomitantly, there has been a drastic decrease in the incidence of new cases of HIV drug resistance in individuals followed longitudinally. For example, of 4477 individuals followed in 2007, there were 36, 92, 71, and 71 cases of PI, NNRTI, NRTI, or 3TC resistance identified, respectively (0.8 to 2% of patients), compared with 7% to15% for these categories in the year 2000. Only 8 newly identified cases of PI resistance have been observed in 2007, and approximately 30 cases for the other resistance categories, for a crude incidence of 0.1 to 0.7%.

Conclusions:  Based on a large cohort of individuals followed longitudinally, our results suggest the effectiveness of HAART therapies has been such that the vast majority (>90%) of treated patients in British Columbia now have either suppressed plasma viral load or drug susceptible HIV in their most recent tests. Efforts to improve accessibility to HAART have the potential to greatly reduce the spread of HIV among susceptible populations without increased risk of drug resistance.