Volume 29-23
1 December 2003

[Table of Contents]

ESTIMATES OF HIV PREVALENCE AND INCIDENCE IN CANADA, 2002

Introduction

1 December, 2003, marks the 16^th annual World AIDS Day. The theme this year is Live and Let Live, a theme that highlights the importance of eliminating stigma and discrimination as major obstacles to effective HIV/AIDS prevention and care. Discrimination causes isolation and marginalization of people who have HIV and AIDS and can prevent them from being provided with or accessing treatment that could reduce suffering and save lives. Stigma and discrimination can also lead to ignorance and fear, which, in turn, lead to ineffective prevention programs and continued infections.

World AIDS Day is a good opportunity to review the progress made in the battle against the epidemic and to bring into focus the remaining challenges. 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), Health Canada, periodically produces estimates of HIV prevalence (those who are living with HIV, including AIDS) and incidence (the number of new infections). This paper presents the estimates for 2002, summarizes the challenges associated with their production, and discusses the implications for HIV prevention and control.

Background

With the recent addition of HIV to the list of reportable diseases in the provinces of Quebec in April 2002 and British Columbia in May 2003, HIV and AIDS are now reportable in all provinces and territories of Canada. Surveillance of HIV and AIDS in Canada is ongoing thanks to the voluntary reporting to CIDPC of positive HIV tests and AIDS diagnoses by all provinces and territories.

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⁾. These data provide a description of people who have been given a diagnosis of HIV or AIDS in Canada; however, there are a number of reasons why surveillance data understate the magnitude of the HIV epidemic. First, surveillance data are subject to delays in reporting, underreporting and changing patterns in HIV testing behaviours (who comes forward for testing). In addition, surveillance data can tell us only about people who have been tested and given a diagnosis of HIV or AIDS and not those who remain untested and whose condition is undiagnosed. Because HIV is a chronic infection with a long latent period, for many people who are newly infected in a given year their infection may not be diagnosed until later years. Consequently, the number of new HIV positive tests that are reported to CIDPC in a given year does not provide an estimate of the number of new HIV infections that occurred in that year, because many of those people will have been infected in a previous year.

Since surveillance data can provide a description only of the diagnosed portion of the epidemic in Canada, the use of additional sources of information, such as research and targeted studies, is required to describe the epidemic among Canadians with both diagnosed and undiagnosed HIV infection. The estimation process brings together all of these types of data; the methods are described in more detail in the following section.

Methods

Methods to estimate prevalence and incidence at the national level are complex and uncertain. The methods used, described next, are similar to the ones that have been used in the USA⁽³⁾ and internationally⁽⁴⁾. 

The four provinces that account for over 85% of the population of Canada and over 95% of reported HIV and AIDS diagnoses are Ontario, Quebec, British Columbia, and Alberta. Separate HIV prevalence and incidence estimates were produced for each of these four provinces for each exposure category: men who have had sex with men (MSM), injecting drug users (IDU), MSM-IDU, heterosexual (heterosexual contact with a person who is either HIV-infected or at risk of HIV, heterosexual contact as the only identified risk, or origin in a country where HIV is endemic) and other (recipients of blood transfusion or clotting factor, perinatal and occupational transmission). Prevalence and incidence were estimated according to a combination of different methods, which incorporate data from a wide variety of sources, such as AIDS case reports, provincial HIV testing databases, population-based surveys, targeted epidemiologic studies and census data. After draft estimates using these methods had been calculated, experts in each of the four provinces, including public health officials, researchers and community representatives, were consulted. On the basis of this valuable feedback, the provisional estimates were improved.

HIV prevalence was estimated using the three methods outlined in Box 1 for each of the four provinces by exposure category. Using Method 1 (direct method), the number of prevalent infections was calculated by multiplying the prevalence rate by the estimated population size (total population for that group). Methods 2 and 3 (indirect methods) were used together to estimate HIV prevalence; they were both based on the number of HIV diagnoses and on information about HIV testing behaviour. In Method 2, the cumulative number of HIV diagnoses less cumulative AIDS deaths was divided by the proportion of the population that had ever been tested for HIV. In Method 3, the number of HIV diagnoses in 2002 was divided by the proportion of the population that had been tested for HIV within the previous year. The result was then added to the cumulative number of HIV diagnoses to the end of 2001, less cumulative AIDS deaths, plus an estimate for 2002 HIV incidence.

Box 2 outlines the method used to estimate HIV incidence. The number of incident infections was derived by multiplying the incidence rate by the estimated population at risk (total population for that group minus those already infected with HIV). The data sources used in these methods are outlined in Box 3.

Box 1.    Number of prevalent HIV infections at the end of 2002, by exposure category

Note: 

1. For the province of Quebec, only Method 1 was used because of limited availability of HIV diagnostic data by exposure category.

2. For the exposure category MSM-IDU, only Method 1 was used because of the limited availability of population-based surveys of risk and testing behaviours specific to this group.

3. An additional method was used to estimate prevalence for the heterosexual exposure category based on antenatal HIV prevalence data and the sex-age distribution of reported HIV diagnoses.

Box 2.    Number of incident HIV infections in 2002, by exposure category

Box 3.    Data sources for prevalence and incidence methods

HIV prevalence and incidence for the other regions in Canada were calculated by extrapolation from national HIV and AIDS surveillance data. Specifically, estimates from the four provinces together with the provincial and territorial distribution of reported HIV and AIDS cases in national surveillance data were used to calculate estimates for the other regions in Canada. 

Estimates of HIV prevalence and incidence among women, Aboriginal people, and people from HIV-endemic countries within the heterosexual exposure category were derived from the overall estimates according to the distributions by sex, ethnic group and exposure category in national HIV and AIDS surveillance data. We also took into account the results of modeling studies from Ontario for people from HIV-endemic countries⁽⁵⁾.

The presentation of the 2002 estimates differs from that of previous years in that more emphasis is placed on ranges, calculated using statistical simulation procedures, rather than point estimates. This reflects the challenges associated with the limited availability of data for this estimation process (see Limitations section).

Results 

HIV Prevalence 

At the end of 2002, an estimated 56 000 (46 000-66 000) people in Canada were living with HIV infection (including AIDS), which represents an increase of about 12% from the point estimate of 49 ,800 at the end of 1999 (Table 1). In terms of exposure category, these prevalent infections in 2002 comprised 32 500 MSM (58% of total), 11 000 IDU (20% of total), 10 000 heterosexuals (18% of total), 2200 MSM-IDU (4% of total), and 300 attributed to other exposures (< 1% of total) (Table 1, Figure 1). The largest absolute increase was in the MSM exposure category, which had 2900 more prevalent infections than in 1999 (10% relative increase). There were an estimated 2000 more prevalent infections in the heterosexual exposure category (25% increase) and 1300 more among IDU (13% increase). 

HIV Incidence 

The estimated number of new infections (incident infections) in Canada continues at approximately the same rate as 3 years ago. An estimated 2800 to 5200 new HIV infections occurred in 2002 as compared with the estimate of 3310 to 5150 in 1999 (Table 2). On examination of the estimates for 2002 by exposure category, it is clear that MSM continue to account for the greatest number of new infections: 1000 to 2000. This represents about 40% of the national total of new infections in 2002 (Figure 1), which is a slight increase from the 38% estimated in 1999. The proportion of new infections among IDU has decreased slightly, from 34% of the total in 1999 to 30% in 2002 (800-1600 new infections in 2002). The proportion attributed to the heterosexual exposure category increased slightly, from 21% in 1999 to 24% in 2002 (600-1300 new infections in 2002). 

Figure 2 shows how the exposure category distribution of new HIV infections has changed since the beginning of the HIV epidemic in Canada. Until 1996, there was a steady increase in the proportion of IDUs among new infections, and since then this proportion has decreased. Conversely, the proportion of MSM among new infections steadily declined until 1996 and has increased since then. The proportion of new infections attributed to the heterosexual exposure category has increased steadily since the beginning of the epidemic. 

Despite the finding that estimated HIV incidence in 2002 is approximately the same as it was in 1999, new HIV diagnoses (positive HIV test reports) have increased recently⁽²⁾. There were 2185 new HIV diagnoses reported to CIPDC in 2001 and 2499 in 2002, representing a 14.4% increase. Such an increase could be due to increased reporting, increased testing, and/or increased infection rates (incidence). Comparing the 2002 incidence estimates with the 1999 estimates indicates that there has not been an increase in HIV incidence, at least not one that was detected by the available data and methods of estimation (although the wide range of uncertainty cannot rule out smaller changes in incidence). Certainly, part of the increase in reported HIV diagnoses is due to the new HIV testing policy for immigrants and refugees implemented by Citizenship and Immigration Canada⁽⁶⁾ on 15 January, 2002. For the subset of this new testing performed in Canada, positive results will be reported on a non-nominal basis through provincial/territorial surveillance systems to the national level.

Ontario also observed an increase in the number of new HIV diagnoses in 2002 as compared with 2001⁽⁷⁾, and the greatest increase was among MSM and applicants for immigrant visas. The increase among visa applicants was entirely related to increased HIV testing. The increase in diagnoses among MSM, however, was only partly due to increased testing, and the remainder of the increase may have been due to a combination of factors. It is possible that there is some increase in incidence in this group or that a subset of MSM, at high risk of HIV, has selectively increased its testing. Further work is required to clarify this matter.

Table 1.    Estimated number of prevalent HIV infections in Canada and associated ranges of uncertainty at the end of 2002 compared with 1999 (point estimates and ranges are rounded)

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

Table 2.    Estimated ranges of uncertainty for number of incident HIV infections in Canada in 2002, compared with 1999 (ranges are rounded)

*    New infections in the Other category are very few and are primarily due to perinatal transmission.

Figure 1. Prevalent and incident HIV infections in Canada by exposure category, Canada, 2002

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

Trends Among Women 

At the end of 2002, there were an estimated 7700 (6500-9000) women living with HIV in Canada (including those living with AIDS), accounting for about 14% of the national total. This represents a 13% increase from the 6800 estimated in 1999. There were 600 to 1200 new HIV infections among women in 2002, representing 23% of all new infections, a finding similar to that in 1999. With respect to exposure category distribution among newly infected women, a slightly higher proportion was attributed to the heterosexual category in 2002 than in 1999 (53% versus 46% respectively). The remainder of new infections among women was attributed to injecting drug use. 

Trends Among Aboriginal People 

Aboriginal people continue to be over-represented in the HIV epidemic in Canada. They represent 3.3% of the Canadian population⁽⁸⁾, and yet an estimated 3000 to 4000 Aboriginal people were living with HIV in Canada in 2002, representing about 5% to 8% of all prevalent HIV infections. This is higher than the 1999 estimate of 2500 to 3000, or about 6% of the total. Aboriginal people accounted for approximately 250 to 450 of the new HIV infections in Canada in 2002, or 6% to 12% of the total, as compared with 9% in 1999. The composition of exposure category among Aboriginal people newly infected in 2002 was similar to that in 1999. The distribution in 2002 was 63% IDU, 18% heterosexual, 12% MSM, and 7% MSM-IDU.

It is important to note that the proportion of newly infected Aboriginal Canadians who are IDU (63%) is much higher than among all Canadians (30%). This indicates the different characteristics of the HIV epidemic among Aboriginal people and underscores the complexity of Canada's HIV epidemic. 

People from HIV-endemic Countries Within the Heterosexual Exposure Category 

The heterosexual exposure category is a diverse group that includes people who have had sexual contact with someone who is either HIV infected or at risk of HIV (such as an IDU or a bisexual male), people who have not identified any risk apart from sexual contact with the opposite sex, and people who were born in a country where HIV is endemic. The term "endemic" refers to a country where heterosexual sex is the predominant mode of HIV transmission and where HIV prevalence is high (primarily countries in sub-Saharan Africa and the Caribbean). 

On the basis of the proportions in positive HIV test reports and reported AIDS cases, it is estimated that in 2002 there were approximately 3700 to 5700 prevalent HIV infections and 250 to 450 incident infections among heterosexual people who were born in a country where HIV is endemic. These numbers represent approximately 7% to 10% of prevalent infections and 6% to 12% of incident infections in Canada. As with the Aboriginal community, people from HIV-endemic countries are over-represented in Canada's HIV epidemic. According to the 2001 Census, approximately 1.5% of the Canadian population were born in an HIV-endemic country⁽⁹⁾. Unfortunately, using the current methods and available data, it is not possible to differentiate between infections acquired abroad and those acquired in Canada. However, CIDPC is currently collaborating with 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. 

Undiagnosed HIV Infections: the Hidden Epidemic 

There have been 52 680 positive HIV tests reported to CIDPC up to 31  December, 2002, since testing began in November 1985. After adjusting for underreporting, we estimated that 57 000 Canadians have been given a diagnosis of HIV, of whom about 18 000 have died. Thus, approximately 39 000 Canadians living with HIV infection in 2002 have had their condition diagnosed. Therefore, of the estimated 56 000 people with prevalent infections in 2002, about 17 000 (13 000-21 000) or 30% were unaware of their HIV infection. 

The number of people in this group is especially difficult to estimate because they are "hidden" to the health care and disease monitoring systems, since they have not yet been tested for and given a diagnosis of HIV infection. This group is particularly important, because until there has been a diagnosis, these people cannot take advantage of available treatment strategies or appropriate counseling to prevent the further spread of HIV. At present, it is not possible to further define this "hidden" group by exposure category or sex, but CIDPC is currently working on addressing this issue. For example, such people may be more likely to be tested late and thus to present with an AIDS diagnosis shortly after their HIV diagnosis. A recent analysis of characteristics associated with late HIV diagnosis among Canadian AIDS cases found that late HIV testers were more likely to belong to an ethnic group other than White and to have been infected by routes other than MSM or IDU (such as heterosexual activity)⁽¹⁰⁾. Such information would be useful to help target programs that are intended to increase awareness of the risk of HIV transmission and improve access to and use of HIV testing.

Limitations

As outlined in the Methods section, the presentation of the 2002 estimates differs from previous years in that more emphasis is placed on ranges rather than point estimates. This change in the presentation was made to reflect the challenges associated with the data available for the estimation process. There was limited availability of new research data on HIV incidence and prevalence, and on the population size of risk groups for each of the four provinces. Furthermore, calculation of the subcategories, such as Aboriginal people and people from HIV-endemic countries, relied on variables in the HIV and AIDS surveillance data that are often poorly reported at the national level. Information on risk factors in HIV/AIDS surveillance data is also incomplete, and this may lead to misclassification of cases to the wrong exposure category. This is a particular problem for cases assigned to the heterosexual exposure category. In the Ontario Laboratory Enhancement Study, a substantial proportion of cases initially categorized as heterosexually acquired on the basis of data from the laboratory requisition were in fact infected by other routes; this was especially true for HIV diagnoses among males⁽¹¹⁾. The estimation of HIV infections in the heterosexual category is complicated by this bias, and we are currently working with our partners in provincial and local health departments to improve the quality of these data and the precision of our estimates.

The reliance on data primarily from the larger cities in the four provinces is also a limitation, and these estimates do not, therefore, necessarily reflect local trends of HIV incidence and prevalence in Canada. 

Discussion 

The methods used to estimate HIV prevalence and incidence make maximum use of a wide variety of data. Producing these national estimates is becoming increasingly difficult because of the current limitations associated with HIV surveillance data and the limited availability of research data specific to HIV incidence and prevalence, and on the population size of risk groups. Limitations associated with HIV surveillance in Canada are currently being addressed in collaboration with our provincial/territorial partners and community groups. Epidemiologic research in Canada needs to be strengthened to provide information that will help improve the estimates. Given the information we have, however, we believe that this is a plausible picture of the state of the epidemic in Canada. 

A total of 56 000 Canadians are living with HIV infection. This number will continue to increase as new infections continue and survival improves as a result of new treatments; this will mean increased care requirements for the future. An estimated 2800 to 5200 new infections occurred in Canada in 2002, about the same as in 1999. Rates of infection remain unacceptably high in all exposure categories, including MSM, which remains the single most affected group. Aboriginal people and people from HIV- endemic countries are over-represented in Canada's HIV epidemic. In addition, the findings of this study highlight the need for specific measures to address the unique aspects of the HIV epidemic within certain subpopulations: injecting drug use is the primary HIV exposure category among Aboriginal people, and heterosexual activity is the main exposure category for women and people from HIV-endemic countries. There also continues to be a sizeable number of people living with but unaware of their HIV infection. Until these people are tested and given a diagnosis, they are unable to take advantage of care and treatment services or to receive appropriate counseling to prevent further spread of HIV.

Greater vigilance is needed if we are going to successfully control the HIV epidemic in Canada. This includes more effective strategies to prevent new infections in all risk groups and to provide services to the increasing number of Canadians living with HIV infection, particularly those who are vulnerable and disadvantaged. In addition, there is an increasing need to address the limited availability of data in order to better understand and monitor the full scope of the HIV epidemic in Canada. 

Acknowledgments 

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 the non-nominal HIV and AIDS surveillance data. In addition, we would like to acknowledge the help and advice of Dena Schanzer, CIDPC, Health Canada. 

References 

1. Health Canada. HIV and AIDS in Canada. Surveillance report to December 31, 2002. Ottawa: Division of HIV/AIDS Epidemiology and Surveillance, Centre for Infectious Disease Prevention and Control, Health Canada, April 2003. 

2. Health Canada. HIV and AIDS in Canada. Surveillance report to June 30, 2003. Ottawa: Division of HIV/AIDS Epidemiology and Surveillance, Centre for Infectious Disease Prevention and Control, Health Canada, November 2003. 

3. Holmberg S. The estimated prevalence of HIV in 96 large US Metropolitan Areas. Am J Public Health 1996;86:642-54. 

4. Walker N, Stanecki KA, Brown T et al. Methods and procedures for estimating HIV/AIDS and its impact: the UNAIDS/WHO estimates for the end of 2001. AIDS 2003;17:1-11. 

5. Remis RS, Whittingham E. The HIV/AIDS epidemic among persons from HIV-endemic countries in Ontario, 1981-1998: situation report. Toronto: Department of Public Health Sciences, University of Toronto, November 1999. 

6. Citizenship and Immigration Canada. Fact sheet 20, Medical testing and surveillance. URL: <http://www.cic.gc.ca/english/pub/fs-medical.html>. 

7. Njihia J, Remis RS, Swantee C et al. Marked increase in first-time HIV diagnoses in Ontario, 2002, 12^th Canadian Association for HIV Research Conference. Can J Infect Dis 2003;14(A), Abstract 209. 

8. Statistics Canada. Aboriginal peoples of Canada: a demographic profile. The Daily. Cat. no. 96F0030XIE2001007, January 2003. 

9. Statistics Canada. Immigrant status and period of immigration (10A) and place of birth of respondent (260) for immigrants and non-permanent residents for Canada, provinces, territories, Census Metropolitan Areas and Census Agglomerations, 20% sample data. Ottawa: Cat 97F0009XCB01002, Census 2001. 

10. Geduld J, Romaguera A, Esteve A et al. Late diagnosis of HIV infection among reported AIDS cases in Canada and Catalonia, Spain. XIV International Conference on AIDS, Barcelona, July 2002 (Abstract WePeC6105). 

11. Remis RS, Swantee C, Major C et al. Enhancing HIV diagnostic data for surveillance of HIV infection: results from the detuned assay to December 2002. 5^th Annual Ontario HIV Treatment Conference, Toronto, Ontario, November 4, 2003.

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