Objective: To estimate HIV incidence density for different exposure categories among people undergoing repeat testing in Ontario, Canada.
Methods: Persons using voluntary, diagnostic HIV testing at least twice were identified by computerized and manual record linkage. In the 1992-2000 period, 980 seroconverters and 340 994 repeat negative testers contributed 936 145 person years (PY) of observation. Incidence density (ID) was calculated according to Kitayaporn et al. Poisson regression was used to evaluate differences in incidence.
Results: Among men who have sex with men (MSM), ID declined between 1992-1996, from 1.23 per 100 PY in 1992 to 0.79 per 100 PY in 1996 [relative risk (RR), 0.86 per year; 95% confidence interval (CI), 0.77-0.96]. Subsequently, ID increased to 1.39 per 100 PY in 1999 (RR, 1.18 per year; 95% CI, 1.05-1.34). In 2000, ID was 1.16 per 100 PY but this decrease was not statistically significantly different from 1999. MSM in their twenties had the highest ID in 1992-1996, but in 1996-2000 MSM in their thirties had the highest risk of infection. Among injecting drug users (IDU), ID decreased from 0.64 per 100 PY in 1992 to 0.14 per 100 PY in 2000 (RR, 0.87 per year; 95% CI, 0.80-0.94). Among heterosexuals, annual incidence remained constant at about 0.03 per 100 PY in 1992-2000.
Conclusions: Increases in ID were identified among MSM from 1996 to 1999. These findings are consistent with other research. Continued vigilance and improved surveillance are needed to better understand and control the epidemic.