Introduction
From the beginning of the HIV epidemic, gay, bisexual, and other men who have sex with men (gbMSM) have been disproportionately impacted [
1]. In 2019, 60% of new diagnoses in Québec were recorded in Montréal, Canada’s second most populated city [
2]. Of the new diagnoses, 70% occurred among men, of which 71% were reported among gbMSM [
3].
In 2017, Montréal became the first Canadian
Fast-Track City, pledging to reach zero new HIV infections by 2030 [
4]. The successful achievement of this ambitious goal largely depends on the implementation of effective city-level public health initiatives to reduce the acquisition and transmission potential for HIV. Understanding the impact of sexual behaviors and specific HIV interventions on the epidemic can aid in the development of relevant HIV elimination plans [
5]. This objective is complicated, however, by the continued evolution of the complex HIV prevention and treatment landscape, which includes: HIV testing (1985) [
6], anti retroviral therapy (ART; 1996) [
7], post-exposure prophylaxis (PEP; 2001) [
8], and pre-exposure prophylaxis (PrEP; 2013) [
8].
The scarcity of gbMSM survey data from the early years of the epidemic adds to the challenges of characterizing HIV epidemics and their complex transmission dynamics using traditional epidemiological methods. Tools such as agent-based models (ABM) –computer simulations of epidemics– are appropriate to understand and characterize past and future transmission dynamics [
9‐
12]. Mathematical models of HIV transmission have been used to describe the drivers (e.g., age, infection stage, knowledge of status, and treatment status [
14,
15]) and dynamics of global and local HIV epidemics. These models have focused on different population subgroups, have been applied to a range of geographical locations, and have studied, both prospectively and retrospectively, the impact of various interventions on HIV dynamics [
11,
16‐
18]. The relevance and applicability of the models is largely dependent on the quality and quantity of the data used to parameterize and inform the population-specific model [
19].
Despite the quantity of available Montréal-based sexual behavioral data, there have been no recent attempts to systematically analyze and triangulate data and surveys on HIV transmission among gbMSM in Montréal to understand evolving transmission dynamics. The purpose of this study was to describe the epidemiological characteristics of the HIV epidemic and understand the sources of HIV acquisition and transmission between 1975 and 2019.
Discussion
Achieving city-level HIV elimination requires a granular understanding of local past and present transmission dynamics, the identification of gaps in the HIV care continuum, and data-driven implementation of appropriate interventions among the populations most impacted by HIV [
45]. Through the development and analysis of a detailed model of HIV dynamics among gbMSM in Montréal, we characterized trends in HIV incidence, prevalence, and mortality, as well as the individual characteristics of HIV acquisitions and transmissions. Our model points to important improvements in epidemiological and continuum of care outcomes, with low HIV incidence in recent years.
At the beginning of the epidemic, the estimated rising mortality rates between 1985 and 1995, combined with some saturation of groups with higher partnering levels, could have played an important role in reducing onward HIV transmission. The latter translated into a decreasing HIV incidence and subsequent fall in prevalence over 1990–2000. The introduction of ART in 1996 is estimated to have led to a sharp drop in mortality, consistent with national estimates of deaths due to AIDS [
1]. With the advent of these life-saving treatments and concomitant incidence reductions, the AIDS epidemic aged [
46]. This was particularly salient with the increasing HIV prevalence among men aged 55 + years. This trend is similar to the national one in Canada where, despite an overall increase in the number of PLHIV, the number of gbMSM PLHIV has stabilized. Nationally, other groups of individuals such as people who inject drugs, Indigenous, and other heterosexual males and females contribute to the increasing national prevalence [
48].
The large incidence ratio between the high and low sexual partnering groups is indicative of the underlying mixing patterns within the population and the increased likelihood of HIV transmission and acquisition among individuals with more sexual partners [
49]. Consequently, the high partnering group experienced a greater reduction in incidence after the introduction of ART. The low/medium sexual partnering groups acquired approximately 20% of new HIV infections but transmitted few. This highlights their smaller role in ongoing transmission of HIV but suggests that renewed HIV prevention efforts that engage men in the low/medium sexual partnering groups are needed to further reduce HIV acquisition. Improving PrEP use among eligible men (e.g., those not using condoms) in these lower sexual activity groups could address such prevention needs. In addition, the model simulations indicate that the majority of transmission and acquisition events took place among gbMSM aged 25–44 years. This result is in line with local HIV surveillance data of new HIV diagnoses [
3].
Our analyses identified opportunities to strengthen HIV prevention. Although we evaluate that diagnosis coverage has achieved high levels (> 90%) in recent years, approximately half of HIV transmission events in 2019 could have occurred from undiagnosed men. With the current low HIV incidence and high HIV testing rates, this undiagnosed fraction has considerably decreased over the last decade. ART coverage among gbMSM in Montréal was also estimated to be high in 2019 (> 90%). However, not all men on ART have a suppressed viral load and our model suggests that close to 1 out of 4 transmission events could be from PLHIV on treatment. There is a short 3–6 month period following ART initiation where viral load suppression has not been achieved and clinical data of gbMSM in Montréal suggest that 93% of those on ART are virally suppressed [
37]. Reflecting this setting of low HIV incidence, high ART coverage, and low AIDS incidence, our model suggests that those in the acute infection stage have contributed a relatively larger proportion of transmission events over the last decade. These estimates are comparable with estimates found among gbMSM in the UK (population attributable fraction: 3–28% in 2014–15) [
11] and Baltimore (8–35% over 2008–17) [
14]. Given the heightened HIV transmissibility during the primary stage of infection [
50], identifying new clusters of infections through the routine use of recency assays [
51], phylogenetic testing [
52], and genetic sequencing [
53] could help achieve HIV elimination. Additionally, continued efforts are necessary to increase timely diagnosis, prompt linkage to ART, the continued promotion of “U = U” (i.e., undetectable equals untransmittable) [
54] to reduce stigma, and sustaining viral load suppression among those who have achieved it.
The study results should be interpreted considering several limitations. First, the paucity of recorded empirical data from the early years of the epidemic (e.g., condom use, sexual behaviors, and HIV testing) necessitated the use of informed assumptions for sexual behaviors. However, cross-validation of model outputs with surveillance data on new HIV diagnoses from 2002 to 2017, suggest that epidemic trends are accurately reflected. Second, the model does not explicitly model HIV transmission through injecting drug use among gbMSM but provincial surveillance data suggest few HIV diagnoses in this combined category. Third, the survey instruments had different eligibility criteria and, in some cases, metrics had slightly different definitions across surveys. However, statistical methods were used to standardize the survey data, ensuring their comparability and improving representativeness. Fourth, ethnicity was not incorporated in the model but studies suggest that in Canada, there are minimal differences in HIV prevalence by race [
55]. Nevertheless, we recognize that some minoritized populations could face distinct barriers to HIV prevention. Finally, the lower HIV incidence in recent years resulted in large uncertainty when performing stratified data analyses.
Conclusions
HIV incidence among Montreal gbMSM has decreased to low levels in recent years. To achieve the goal of HIV elimination, continued surveillance efforts and the strategic implementation of combination HIV prevention strategies, including increased frequency of HIV testing among high sexual partnering individuals. By characterizing the HIV dynamics, it was possible to identify the population subgroups most impacted by HIV and the need to further improve ART retention. The implementation of interventions that focus on gbMSM aged 25–45 years, and gbMSM in the high partnering group could help to achieve HIV elimination among gbMSM in Montréal.
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