Background
There has been a marked rise in new diagnoses in Montreal from 140 cases in 2021 to 310 in 2022. Population-level viral phylogenetics can provide a unique molecular framework to better ascertain the impact of migration on the introduction and crossover of subtype B and non-B subtype epidemics among Men having Sex with Men (MSM) and heterosexual (HET) groups risk populations. This study combined viral phylogenetics with epidemiological data from two clinical sites to characterize evolving trends shaping epidemic growth between 2016 and 2023.
Methods
Viral sequences and test requisition data from newly diagnosed persons were accrued from the Quebec drug resistance testing program. Viral clustering at strict (1.0%) and relaxed (2.5%) genetic distance thresholds (2.5%), mapped transmission networks shaping subtype B epidemics among MSM (male only singletons and clusters) and HET (female singletons mixed gender clusters) groups. Non-nominative clinical data (age, sex, gender, sexual orientation, country of birth and date of arrival) was obtained with consent from diagnosed persons seen at Clinique l’Actuel (n= 190, 2016─2020) and the McGill University Health Centre (n= 49, 2021─2023).
Results
Improved treatment-as-prevention paradigms have led to 53% declines in the incident infections in 2014─2020 (n=590) vs. 2007─2013 (n=1171). However, epidemic control was thwarted by the episodic development of large cluster outbreaks. Between 2014 and 2020, 36 clusters added median 11 (9-22 IQR) members, contributing to 665 incident infections. Of concern, 26% of recent migrant MSM acquired their infection in the province, segregating with 14 large clusters. In addition, ten non-B clusters arose among MSM (24% of cases) between 2017─2023), with median 9 [8-12.5 IQR} members per cluster, leading to 80 incident infections. In contrast, large non-B subtype HET clusters rarely occurred (4.1% of cases).
Conclusions
These findings emphasize the need to target treatment and prevention measures, including preexposure prophylaxis, for high at-risk newcomers to Quebec and increasingly younger populations.