HIV/AIDS Epi Update - May 2004

National HIV Prevalence and Incidence Estimates for 2002

Introduction

This Epi Update outlines the estimates of the total number of Canadians who were living with HIV infection at the end of 2002 (prevalence) and the number of individuals who became newly infected in 2002 (incidence). It updates estimates produced in 1999. National estimates of HIV prevalence and incidence are an integral part of the work carried out by the Centre for Infectious Disease Prevention and Control. They are used as a tool to monitor the HIV epidemic and to help evaluate and guide prevention efforts, and they are part of ongoing risk assessment and management work conducted by the Centre.

Methods

Methods to estimate prevalence and incidence at the national level are complex and uncertain. The methods used are described below and have been given in detail previously.¹ They are similar to methods that have been used in the USA² and internationally.³

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 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). Methods to estimate prevalence and incidence are based on a combination of different methods and 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 the calculation of draft estimates by means of these methods, 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 by using the three methods 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.

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).

Results

Prevalence Estimates

More people are living with HIV infection (prevalent infections). At the end of 2002, there were an estimated 56,000 (46,000-66,000) people in Canada 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), 2,200 MSM/IDU (4% of total), and 300 attributed to other exposures (< 1% of total) (Table 1).

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	MSM-IDU	IDU	Heterosexual	Other	Total
2002	32,500 (26,000-39,000)	2,200 (1,500-3,000)	11,000 (8,500-13,500)	10,000 (7,000-13,000)	300 (200-400)	56,000 (46,000-66,000)
1999	29,600 (26,000-33,400)	2,100 (1,700-2,600)	9,700 (8,100-11,800)	8,000 (6,300-10,100)	400 (330-470)	49,800 (45,000-54,600)
MSM: men who have sex with men; IDU: injecting drug users; heterosexual: heterosexual contact with a person at risk of HIV, origin in a country where HIV is endemic or heterosexual as the only identified risk; Other: recipients of blood or blood products, perinatal and occupational transmission

Incidence Estimates

The number of new infections (incident infections) continues at approximately the same rate as three years ago. In Canada, there were an estimated 2,800 to 5,200 new HIV infections in 2002 compared with the estimate of 3,310 to 5,150 in 1999 (Table 2). Examining the estimates for 2002 by exposure category, it is clear that MSM continue to account for the greatest number of new infections, 1,000 to 2,000. This represents about 40% of the national total of new infections, which is a slight increase from the 38% estimated in 1999 (Figure 1). The proportion of new infections attributable to IDU has decreased slightly, from 34% of the total in 1999 to 30% in 2002 (800-1,600 new infections in 2002). The proportion attributed to the heterosexual exposure category increased slightly, from 21% in 1999 to 24% in 2002 (600-1,300 new infections in 2002).

Table 2. Estimated ranges of uncertainty for number of incident HIV infections in Canada in 2002, compared with 1999 (ranges are rounded)
	MSM	MSM-IDU	IDU	Heterosexual	Other*	Total
2002	1,000-2,000	150-350	800-1,600	600-1,300	< 20	2,800-5,200
1999	1,190-2,060	190-360	1,030-1,860	610-1,170	< 20	3,310-5,150
*New infections in the Other category are very few and are primarily due to perinatal transmission.

Figure 1 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 new infections attributed to IDU, and since then this proportion has decreased. Conversely, the proportion attributed to MSM 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.

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Figure 1. Estimated exposure category distributions (%) among new HIV infections in Canada, by time period

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Trends among Women

At the end of 2002, there were an estimated 7,700 (6,500-9,000) 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 6,800 estimated in 1999. There were 600 to 1,200 new HIV infections among women in 2002, representing 23% of all new infections, a finding similar to that in1999. With respect to the exposure category distribution among newly infected women, a slightly higher proportion of new infections was attributed to the heterosexual category in 2002 compared with 1999 (53% versus 46% respectively). The remainder of new infections among women was attributable to IDU.

Trends among Aboriginal Persons

In 2002, it was estimated that approximately 3,000 to 4,000 Aboriginal persons were living with HIV in Canada. This represents about 5% to 8% of all prevalent HIV infections, compared with the 1999 estimate of about 6% of the total, or 2,500 to 3,000 persons. Aboriginal persons accounted for approximately 250 to 450 of the new HIV infections in Canada in 2002, or 6% to 12% of the total, compared with 9% in 1999. The composition of exposure category among Aboriginal persons 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.

Persons from HIV-endemic Countries within the Heterosexual Exposure Category

As already outlined, the heterosexual exposure category is a diverse group that includes those who have had sexual contact with a person at risk of HIV (such as an IDU or a bisexual male), those who were born in a country where HIV is endemic, and those who have not identified any risk apart from sexual contact with the opposite sex. On the basis of the proportions in positive HIV test reports and reported AIDS cases, it is estimated that in 2002 there were approximately 3,700 to 5,700 prevalent HIV infections and 250 to 450 incident infections among persons who were born in a country where HIV is endemic. These numbers represent approximately 7% to 10% of total prevalent infections and 6% to 12% of total incident infections in Canada. We are currently collaborating with provincial/ territorial partners, researchers and community groups to explore ways to better understand the current status and trends of HIV infection in this group.

Undiagnosed HIV Infections: the Hidden Epidemic

Using methods described elsewhere,1,2 it was estimated that of the 56,000 prevalent infections in 2002, about 17,000 (13,000-21,000) or 30% were unaware of their HIV infection. The number of persons 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 HIV infection and their condition diagnosed. This group is particularly important because until their infection has been diagnosed, they cannot take advantage of available treatment strategies or appropriate counselling to prevent the further spread of HIV.

Comments

The methods that were 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 existing limitations associated with HIV surveillance data and the limited availability of research data specific to HIV incidence, prevalence and 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. Epidemiological research in Canada needs to be strengthened to provide information that will help improve the estimates. To reflect the challenges associated with the data, the presentation of the 2002 estimates differs from previous years, in that more emphasis is placed on ranges rather than point estimates, especially in the case of incidence, for which data on recent trends are more limited. Given the information we have, however, we believe that this is an accurate picture of the state of the epidemic in Canada.

Available data show that more Canadians are living with HIV infection, and the overall rate of new infections in 2002 was approximately the same as in 1999. MSM continue to be the most affected group, and new infections among IDU continue to decline slightly. Infections attributed to the diverse heterosexual exposure category continue the gradual increase seen previously. The reasons for these trends need to be better understood. It is clear that the number of new infections in all exposure categories remains unacceptably high. Findings also indicate that there are a large number of people in Canada who are unaware of their HIV infection and that Aboriginal persons are still overrepresented in terms of HIV infections in Canada.

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.

References

1. Geduld J, Gatali M, Remis RS, Archibald CP.
   Estimates of HIV prevalence and incidence in Canada, 2002. CCDR 2003;29:197-206.
2. Holmberg S.
   The estimated prevalence of HIV in 96 large US metropolitan areas. Am J Public Health 1996; 86:642-54.
3. Walker N, Stanecki KA, Brown T, Stover J, 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.

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