Progression of HIV infection among injecting drug users: indications for a lower rate of progression among those who have frequently borrowed injecting equipment.

To study markers of progression in a cohort of HIV-infected intravenous drug users (IDU). A prospective epidemiologic study. We studied progression of HIV infection among 126 IDU attending the Municipal Health Service in Amsterdam. Progression was defined as a decline of the CD4 cell count to < 200 x 10(6)/l on two consecutive follow-up visits or AIDS. Using Cox modelling, the following baseline variables were predictive of progression. Enhanced progression was associated with: age > 30 years [relative hazard (RH), 7.7 [95% confidence intervals (CI), 1.7-36.0]], core antibody negativity [RH, 5.3 (95% CI, 1.6-17.6)], CD4 cell count [for CD4 cells 350-500 x 10(6)/l, RH, 1.38 (95% CI, 0.37-5.16); for CD4 cells 200-350 x 10(6)/l, RH, 9.20 (95% CI, 2.73-31.05) compared with a CD4 count > 500 x 10(6)/l]. A lower rate of progression was associated with borrowing used injecting equipment. IDU who reported borrowing injecting equipment between 1980 and baseline 10-99 times or > 99 times had a RH of 0.44 (95% CI, 0.22-0.88) and 0.19 (95% CI, 0.03-0.37), respectively, compared with IDU who had borrowed < 10 times. p24 antigen positivity was more predictive than core antibody negativity in a model with time-dependent variables, the relative risk for p24 antigen-positive participants was 3.5 (95% CI, 1.3-9.3). Additional analysis of progression to AIDS in a larger group of IDU showed comparable results with regard to the effect of borrowing on progression. Our observation that those IDU who reported borrowing injecting equipment most frequently appeared to have the lowest rate of progression, corrected for some sources of potential confounding, requires further epidemiologic confirmation and extended laboratory studies since other sources of bias might have been present. Baseline CD4 count, age and core antibody or p24 antigen were predictive of progression in IDU. We wish to emphasize that our results do not imply that borrowing should be encouraged, but may have implications for our understanding of HIV pathogenesis.