Prevalence and incidence are two basic measures of disease frequency used to inform our response to the HIV epidemic. Prevalence is typically defined as the total number of people living with HIV in a year, while incidence is the number of new infections that occur over a year.
New HIV diagnoses (i.e., case reports) are the foundation of HIV surveillance in British Columbia; however, because they only include individuals who are tested and only count them at the time of diagnosis, these data are an imperfect substitute for HIV incidence. Case surveillance for HIV is also limited in its ability to generate prevalence counts, since prevalence must account for historical data and data on migration and death. For these reasons, models are used to incorporate various sources of knowledge about the HIV epidemic and produce estimates of HIV prevalence and incidence in the population.
The Public Health Agency of Canada (PHAC) has recently released updated national estimates of HIV prevalence and incidence based on modelling and multiple data sources, which we compare here to PHAC estimates for the province of British Columbia.
HIV prevalence has increased over time, with an estimated 71,300 people living with HIV in Canada (range 58,600-84,000) and 11,700 (range 9,400-14,000) in BC, in 2011. This rise is explained by the accumulation of incident cases and decreases in HIV-related deaths. In 2011, the incidence estimate decreased slightly to 3,175 infections (range 2,250-4,100) in Canada, 380 (range 260-500) in BC.
Looking in more detail at estimates of HIV prevalence and incidence by major exposure category signals differences in the profile of the Canadian and British Columbian HIV epidemics. The percentage of estimated prevalent and incident cases in Canada by exposure category is shown below alongside the same figures for BC. This comparison suggests that relative to the Canadian epidemic, prevalent cases in BC are more likely to have acquired HIV through injection drug use, while incident cases are more likely to be gay men or other men who have sex with men. Persons from HIV-endemic countries are estimated to be less common in both prevalent and incident HIV cases in BC as compared with the Canadian epidemic.
MSM=men who have sex with men; IDU=people who use injection drugs; heterosexual/non-endemic=people who report heterosexual contact (excluding those with origin in a country where HIV is endemic); heterosexual/ endemic=people who report heterosexual contact and origin in a country where HIV is endemic.
National estimates are presented in more detail in Estimates of HIV Prevalence and Incidence in Canada, 2011, at the PHAC website. Provincial estimates are shown in the BC Centre for Disease Control Annual HIV Surveillance Report, 2011.
Mark Gilbert, Physician Epidemiologist, Clinical Prevention Services, BCCDC
Public Health Agency of Canada (PHAC)