ARCHIVED - Estimates of HIV prevalence and incidence in Canada, 2005

1 August 2006

Volume 32

Number 15

D Boulos, MSc (1), P Yan, PhD (1), D Schanzer, MSc (1), RS Remis, MD, MPH (2), CP Archibald, MD, FRCPC (1)

1 Centre for Infectious Disease Prevention and Control, Public Health Agency of Canada, Ottawa, Ontario
2 Department of Public Health Sciences, University of Toronto, Toronto, Ontario

Introduction

Estimating HIV prevalence and incidence is a task undertaken around the world to monitor the HIV epidemic and to help assess the effectiveness of prevention efforts. The XVI International AIDS Conference will be held in Toronto on August 13-18, 2006, and the conference theme, "Time to Deliver", underscores the urgency to ensure that effective HIV prevention and treatment programs are made available to communities around the world. It also highlights the need for the appropriate evaluation of the impact of these programs and the production of HIV estimates is an integral part of this process.

As part of its mandate to monitor the epidemiology and trends of HIV/AIDS in Canada, the Centre for Infectious Disease Prevention and Control (CIDPC), Public Health Agency of Canada (PHAC), periodically produces estimates of national HIV prevalence (those who are living with HIV, including AIDS) and incidence (the number of new infections). These estimates help guide the work done by PHAC and other federal departments under the Federal Initiative to Address HIV/AIDS in Canada.This paper presents the estimates for 2005, summarizes the challenges associated with their production, and highlights the implications for HIV prevention and control.

Background

HIV and AIDS surveillance data are presented regularly in a semi-annual report HIV and AIDS in Canada, published each April(1), and November(2), providing a description of persons diagnosed with HIV or AIDS in Canada. However, surveillance data understate the magnitude of the HIV epidemic because such data are subject to reporting delays, underreporting and changing patterns in HIV testing behaviours (who comes forward for testing); surveillance data also do not include individuals who remain untested and undiagnosed. Since HIV is a chronic infection with a long incubation period, many newly infected persons may only be diagnosed in the years after infection. Consequently, the number of new HIV positive tests reported to CIDPC in a given year does not estimate the new HIV infections that occurred in that year because many will have been infected in earlier years.

Since surveillance data can only describe the diagnosed portion of the epidemic, modelling and additional sources of information are required to describe the epidemic among both diagnosed and undiagnosed Canadians. The methods used to estimate HIV prevalence and incidence at the national level bring together all available data and are described in the next section.

Methods

We used multiple methods to estimate national HIV prevalence and incidence in 2005, including the workbook method⁽³⁾,an iterative spreadsheet model⁽⁴⁾ and two statistical modelling methods^(5,6). The workbook method multiplies an estimated prevalence or incidence rate by an estimated population size, the statistical models back-calculate estimates of HIV incidence by relating the timing of HIV positive testing with timing of HIV infection and testing behaviour, and the iterative spreadsheet model incorporates elements of the other two methods. The methods were used to generate separate estimates of HIV prevalence and incidence in Ontario, Quebec, British Columbia and Alberta. These provinces together account for over 85% of the population of Canada and over 95% of reported HIV and AIDS diagnoses. Estimates were further sub-classified according to the following exposure categories: men who have had sex with men (MSM), injecting drug users (IDU), MSM-IDU, heterosexual/ endemic (non-IDU heterosexual with origin in a country where heterosexual sex is the predominant mode of HIV transmission and HIV prevalence is high [primarily countries in sub-Saharan Africa and the Caribbean])^(1,2), heterosexual/non-endemic (heterosexual contact with a person who is either HIV-infected or at risk for HIV, or heterosexual as the only identified risk) and other (recipients of blood transfusion or clotting factor, perinatal and occupational transmission). For some exposure category and province combinations, the modelling methods were not able to produce estimates and in these cases, surveillance data were used to partition out the most likely distribution of the provincial estimates among exposure categories. The results of the different methods were averaged to obtain exposure category-specific prevalence and incidence estimates for each of the four provinces noted above.

HIV prevalence and incidence estimates for the remainder of Canada were extrapolated from these four provinces using national HIV surveillance data. These national surveillance data were obtained from the national HIV and AIDS surveillance reporting system^(1,2) with enhancements from two sources: the Laboratory Enhancement Study in Ontario⁽⁷⁾,which has more complete information on exposure category of HIV cases, and recent published⁽⁸⁾ and unpublished surveillance data from Quebec on exposure category breakdowns of cases newly diagnosed with HIV during 2002 to 2005.

National estimates of HIV prevalence and incidence for years before 2005 were obtained by determining the 2005 estimates as described above and then using the results from modelling to describe the past distributions of HIV prevalence and incidence relative to the 2005 estimate. Bounds of uncertainty for the national HIV estimates were developed based on a conservative consideration of results from a variety of scenarios. Estimates published in this report for years before 2005 replace all previous estimates that we have published concerning HIV prevalence and incidence in Canada because new data and methods have permitted an improved analysis of the epidemic and more reliable estimates.

Estimates of HIV prevalence and incidence among women and Aboriginal persons were derived from the overall estimates using the distributions of reported gender and Aboriginal status by exposure category in the national HIV and AIDS surveillance data. The number of undiagnosed individuals living with HIV infection was computed as prevalence less cumulative HIV diagnoses, adjusted for under- and duplicate reporting and mortality.

Results

HIV prevalence

At the end of 2005, an estimated 58,000 (48,000 to 68,000) people in Canada were living with HIV infection (including AIDS) which represents an increase of about 16% from the 2002 estimate of 50,000 (Table 1). In terms of exposure category, prevalent infections in 2005 were comprised of 29,600 MSM (51%), 9,860 IDU (17%), 8,620 heterosexual/non-endemic (15%), 7,050 heterosexual/endemic (12%), 2,250 MSM-IDU (4%), and 400 attributed to other exposures (< 1% ) (Table 1). The largest absolute increase was among the MSM exposure category with 3,400 more prevalent infections since 2002 (13% increase). There were an estimated 1,670 more prevalent infections in the heterosexual/ non-endemic exposure category (24% increase), 1,370 more among the heterosexual/endemic category (24% increase) and 960 more among IDU (11% increase).

Table 1. Estimated number of prevalent HIV infections in Canada and associated ranges of uncertainty at the end of 2005 and 2002 (point estimates and ranges are rounded)

*Totals were rounded to the nearest 1,000. Unrounded totals were 57,780 for 2005 and 49,980 for 2002 which were used to compute percentages.

MSM: men who have sex with men; IDU: injecting drug users; Heterosexual/non-endemic: heterosexual contact with a person who is either HIV-infected or at risk for HIV or heterosexual as the only identified risk; Heterosexual/endemic: origin in a country where HIV is endemic; Other: recipients of blood transfusion or clotting factor, perinatal and occupational transmission

HIV prevalence: past trends

Prevalent infections (Figure 1) rose steadily during the 1980's, corresponding to the initial rise in HIV infection in the Canadian population, mainly among MSM. This rise reached a plateau in the early to mid-1990's, likely as a result of both increased mortality and effective prevention programs. Prevalent infections began to rise again in the late 1990's due to new treatments improving survival of HIV-infected individuals combined with continuing new infections.

HIV incidence

The number of new HIV infections in Canada in 2005 has not decreased and may have increased slightly compared to 2002. An estimated 2,300 to 4,500 new HIV infections occurred in 2005 compared with 2,100 to 4,000 in 2002 (Table 2). Examining the estimates by exposure category, MSM continues to comprise the greatest number of new infections, 1,100 to 2,000 (45%) compared to 900 to 1,700 (42%) in 2002 (Table 2). The number of new infections estimated among IDU has decreased from a range of 400 to 700 (19%) in 2002 to 350 to 650 (14%) in 2005. For the heterosexual/non-endemic exposure category, the range increased from 450 to 850 (21%) in 2002 to 550 to 950 (21%) in 2005.

Persons from HIV-endemic countries continue to be over- represented in Canada's HIV epidemic. New infections attributed to the heterosexual/endemic exposure category increased slightly from a range of 300 to 600 (15%) in 2002 to 400 to 700 (16%) in 2005, yet according to the 2001 Census, approximately 1.5% of theCanadianpopulationwereborninanHIV-endemic country⁽⁹⁾. Therefore, in 2005, the estimated infection rate among individuals from HIV endemic countries was at least 12.6 times higher than among other Canadians. With the current methods and available data, it is not possible to differentiate infections acquired abroad from those acquired in Canada. CIDPC is currently collaborating with other government departments, provincial/territorial partners, researchers and community groups to develop methods and obtain data to better understand the current status and trends of HIV infection in this group.

Table 2. Estimated ranges of uncertainty for number of incident HIV infections in Canada in 2005 and 2002 (ranges are rounded)

MSM: men who have sex with men; IDU: injecting drug users; Heterosexual/non-endemic: heterosexual contact with a person who is either HIV-infected or at risk for HIV or heterosexual as the only identified risk; Heterosexual/endemic: origin in a country where HIV is endemic; Other: recipients of blood transfusion or clotting factor, perinatal and occupational transmission. *New infections in the Other category are very few, and are primarily due to perinatal transmission.

Figure 1. Estimated number of prevalent HIV infections in Canada, including range of uncertainty, by year

HIV incidence: past trends

The distribution of new HIV infections by exposure category has changed since the beginning of the HIV epidemic in Canada (Figure 2). The proportion of MSM among new infections steadily declined until 1996 and has increased since then while there was a steady increase in the proportion of IDUs among new infections until 1996 and then a decrease. The proportions of new infections attributed to the heterosexual/endemic and non-endemic exposure categories have increased steadily since the beginning of the epidemic.

Figure 3 presents the uncertainty range for estimated HIV incidence over time. New infections peaked during 1984-1985 and this was associated primarily with the MSM population (Figure 2). The number of incident infections decreased steadily after 1985 until the early 1990's and was followed by a slight secondary peak during 1996 and 1997 that was associated with high infection rates among the IDU population (Figure 2). Incident infections may have increased somewhat since the late 1990's, but there is a great deal of uncertainty associated with recent incidence estimates and if present, this increase is much less than that seen in the early 1980s. At any rate, it can be stated with more certainty that the recent trend in incidence does not appear to be decreasing.

In national HIV surveillance data, new positive HIV test reports increased from 2001 to 2002 and then changed very little over the period from 2002 to 2005^(1,2). New diagnoses reported to CIDPC were 2,178 in 2001, 2,494 in 2002, 2,497 in 2003, 2,535 in 2004 and 2,483 in 2005. Some, but likely not all, of this increase between 2001 and subsequent years was due to the new HIV testing policy for immigrants and refugees implemented by Citizenship and Immigration Canada⁽¹⁰⁾ on 15 January, 2002.

Figure 2. Estimated exposure category distributions (%) of new HIV infections in Canada, by time period

Figure 3. Estimated range of uncertainty (represented by vertical bars) in the number of new HIV infections in Canada, for selected years of infection

Trends among women

At the end of 2005, there were an estimated 11,800 (10,000 to 13,500) women living with HIV (including AIDS) in Canada, accounting for about 20% of the national total. This represents a 23% increase from the 9,600 estimated for 2002. There were 620 to 1,240 new HIV infections among women in 2005, representing 27% of all new infections. For 2002, it was estimated that 490 to 970 new HIV infections were among women, comprising about 24% of all new infections. With respect to exposure category, a slightly higher proportion of new infections among women was attributed to the heterosexual category in 2005 compared to 2002 (76% versus 74% respectively). The remainder of new infections among women was attributed to IDU.

Trends among Aboriginal persons

Aboriginal persons continue to be over-represented in the HIV epidemic in Canada. They represent 3.3% of the Canadian population⁽¹¹⁾ and yet an estimated 3,600 to 5,100 Aboriginal persons were living with HIV in Canada in 2005, representing about 7.5% of all prevalent HIV infections. This is higher than the estimated 3,100 to 4,400 for 2002, but represents the same proportion (7.5%). Aboriginal persons comprised approximately 200 to 400 of the new HIV infections in 2002 and 2005, which is about 9% of the total for 2005 and 10% for 2002. Therefore, in 2005, the overall infection rate among Aboriginal persons was about 2.8 times higher than among non-Aboriginal persons. The distribution of newly infected Aboriginal persons among exposure categories in 2005 was 53% IDU, 33% heterosexual, 10% MSM and 3% MSM-IDU, which is unchanged from 2002.

The proportion of new HIV infections in 2005 due to IDU among Aboriginal Canadians (53%) is much higher than among all Canadians (14%). This highlights the uniqueness of the HIV epidemic among Aboriginal persons and underscores the complexity of Canada's HIV epidemic.

Undiagnosed HIV infections: the hidden epidemic

There have been 60,160 positive HIV tests reported to CIDPC since testing began in November 1985 to 31 December, 2005, which translates to about 62,800 after adjusting for under-reporting and duplicates. Of these, we further estimate that approximately 20,800 have died. Thus, 42,000 Canadians living with HIV infection in 2005 have been diagnosed. By subtracting this number from the estimated number of prevalent infections in 2005 (58,000 or 57,800 before rounding), we estimate that about 15,800 people (11,500 to 19,500) or 27% were unaware of their HIV infection. This compares with an estimated 14,400 (10,700 to 17,900) or 29% who were living and unaware of their HIV infection in 2002.

The size of this group is especially difficult to estimate because its members are "hidden" to the health care and disease monitoring systems. It is important to reach this group since undiagnosed individuals cannot take advantage of available treatment strategies or appropriate counselling to prevent the further spread of HIV. Currently, it is not possible to further define this "hidden" group by exposure category or gender, but CIDPC is working to address this issue. For example, among AIDS cases in Canada, persons with a late HIV diagnosis are more likely to belong to a non-White ethnic group and to have been infected by routes other than MSM or IDU (such as by heterosexual activity)⁽¹²⁾. Such information can assist in targeting programs to increase awareness of the risk of HIV transmission and improve access to and use of HIV testing.

Limitations

The 2005 estimates differ from previous years in that more emphasis has been placed on a combination of methods. However, the amount of data available was not always sufficient for the modelling to estimate exposure category-specific numbers for all provinces; in these cases, HIV and AIDS surveillance data were used to extrapolate the additional numbers. The workbook method was heavily dependent on the representativeness of available data and on the assumptions made for groups where recent data were lacking.

Estimates for the Aboriginal subpopulation relied on ethnic variables in the HIV and AIDS surveillance data that are not completely reported at the national level. Information on risk factors in surveillance data was also incomplete and this may have led to the misclassification of some cases. Furthermore, insufficient information was available to distinguish infections acquired outside Canada from those acquired within. Therefore, incidence as used in this report refers to a new infection appearing in Canada, either through transmission within Canada or the arrival of an HIV positive individual. CIDPC is currently working with its partners to obtain data that would allow for the separate modelling of domestically acquired infections and the subsequent addition of newly arrived infections to these estimates.

These national estimates do not necessarily reflect local trends in HIV prevalence and incidence. The estimates do not address all populations affected by the HIV/AIDS epidemic in Canada (for example, prisoners) and the estimates are not broken down by age.

Discussion

The methods used to estimate HIV prevalence and incidence incorporated a wide variety of data. Additional sources of surveillance data were available from Ontario and Quebec that provided greater clarity to the characteristics of the epidemic in these provinces. Statistical modelling methods were used for the first time, making optimal use of the national HIV surveillance data. For future estimates, we plan to make increased use of tests to identify recent infections among diagnosed cases and to utilize more results from targeted studies among high-risk populations. Despite the limitations noted, we believe this is a plausible picture of the state of the epidemic in Canada.

Approximately 58,000 Canadians were estimated to be living with HIV infection. This number will likely increase as new infections continue and survival improves due to new treatments, which will mean increased future care requirements. An estimated 2,300 to 4,500 new infections occurred in Canada in 2005, slightly higher than was estimated for 2002. However, the increase cannot be stated with certainty due to the level of precision associated with the estimates; a firmer conclusion is that overall incidence is not decreasing.

This recent trend among MSM and MSM-IDU is associated with increases in risky sexual behaviour. The causes of this increase are complex and may include decision-making based on false assumptions about a partner's HIV status, dissatisfaction and difficulties with condom use, feelings of marginalization, depression and the choice to not use condoms as a gesture of commitment to a partner. Additionally, increases in risky sexual behaviour may be facilitated by use of recreational drugs and, among young MSM, the lack of direct experience with AIDS cases. Among the heterosexual exposure category, the observed trend is likely a result of the general evolution and spread of the epidemic as well as a recent change in the Citizenship and Immigration Canada policy on testing immigrants and refugees⁽¹⁰⁾, which has resulted in more diagnoses. The decrease among IDU likely results, at least in part, from effective prevention programming and shifting patterns of drug injecting practices.

Aboriginal people and persons from HIV-endemic countries continue to be over-represented in Canada's HIV epidemic, highlighting the need for specific measures to address the unique aspects of the HIV epidemic within certain subpopulations. For example, IDU is the main HIV exposure category among Aboriginal persons while heterosexual activity is the main risk for women and persons from HIV-endemic countries. There also continues to be a sizeable number of people unaware of their HIV infection. Until these people are tested and diagnosed, they cannot take advantage of appropriate care and treatment services or receive counselling to prevent further spread of HIV.

To successfully control the HIV epidemic in Canada, more effective strategies are needed to prevent new infections and provide services for all of the vulnerable populations identified in the Federal Initiative to Address HIV/AIDS in Canada. In addition, there is an increasing need to improve the availability and quality of data to better understand and monitor the full scope of the HIV epidemic in Canada.

Acknowledgements

The authors would like to thank public health officials, HIV researchers and community representatives in the provinces of Ontario, Quebec, British Columbia and Alberta for their support and collaboration in producing these estimates. We would also like to thank the provincial and territorial HIV/AIDS coordinators, laboratories, health care providers, and reporting physicians for providing HIV and AIDS surveillance data.

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