Monkeypox in Montréal: Epidemiology, Phylogenomics, and Public Health Response to a Large North American Outbreak

BACKGROUND Monkeypox, a viral zoonotic disease, is causing a global outbreak outside of endemic areas. OBJECTIVE To characterize the outbreak of monkeypox in Montréal, the first large outbreak in North America. DESIGN Epidemiologic and laboratory surveillance data and a phylogenomic analysis were used to describe and place the outbreak in a global context. SETTING Montréal, Canada. PATIENTS Probable or confirmed cases of monkeypox. MEASUREMENTS Epidemiologic, clinical, and demographic data were aggregated. Whole-genome sequencing and phylogenetic analysis were performed for a set of outbreak sequences. The public health response and its evolution are described. RESULTS Up to 18 October 2022, a total of 402 cases of monkeypox were reported mostly among men who have sex with men (MSM), most of which were suspected to be acquired through sexual contact. All monkeypox genomes nested within the B.1 lineage. Montréal Public Health worked closely with the affected communities to control the outbreak, becoming the first jurisdiction to offer 1 dose of the Modified Vaccinia Ankara-Bavarian Nordic vaccine as preexposure prophylaxis (PrEP) to those at risk in early June 2022. Two peaks of cases were seen in early June and July (43 and 44 cases per week, respectively) followed by a decline toward near resolution of the outbreak in October. Reasons for the biphasic peak are not fully elucidated but may represent the tempo of vaccination and/or several factors related to transmission dynamics and case ascertainment. LIMITATIONS Clinical data are self-reported. Limited divergence among sequences limited genomic epidemiologic conclusions. CONCLUSION A large outbreak of monkeypox occurred in Montréal, primarily among MSM. Successful control of the outbreak rested on early and sustained engagement with the affected communities and rapid offer of PrEP vaccination to at-risk persons. PRIMARY FUNDING SOURCE None.