Rapid risk assessment: Clade Ib mpox virus, public health implications for Canada due to increased global community transmission

Assessment completed: November 3, 2025 (with data as of October 28, 2025)

On this page

• Reason for the assessment
• Risk question
• Risk statement
• Event summary
• Considerations for pathogens with pandemic potential
• Risk assessment details
• Limitations, knowledge gaps, and uncertainties
• Proposed actions
• Reassessment
• Methods
• Acknowledgements
• References

Reason for the assessment

Reports of autochthonous transmission of clade Ib mpox virus (MPXV) in the United States (U.S.), Spain, Italy, the Netherlands, Portugal, and Malaysia, combined with wastewater detections of clade I (subclade investigation ongoing) in Canada, raise concern for potential importation and establishment of community transmission within Canadian populations, particularly those at increased risk of exposure (i.e., gay, bisexual, and other men who have sex with men (gbMSM)).

Risk question

What is the likelihood and impact of clade Ib MPXV importation leading to at least one locally acquired case in Canada in the next ten weeks?

Risk statement

The overall risk of clade Ib MPXV importation leading to at least one locally acquired case for the general population in Canada is low, however for gbMSM sexual networks the level of risk is higher, given evidence that recent transmission outside of the African region is predominantly occurring among these populations. The uncertainty for both populations is moderate given limited evidence on transmission patterns in countries outside of the African region.

In line with our previous risk assessment ^{Footnote 1}, the likelihood of clade Ib MPXV importation is high, driven by increased autochthonous transmission in regions with large travel ties to Canada. The likelihood that imported clade Ib MPXV cases will lead to at least one locally acquired case in the next ten weeks is very low for the general population, but high within gbMSM sexual networks. Preliminary evidence suggests clade Ib transmission in the U.S., Europe and Asia is occurring primarily among gbMSM through sexual contact, with spread beyond intimate contacts and sexual networks remaining rare.

The impact of clade Ib MPXV infection on an affected individual is estimated to be minor based on early evidence suggesting milder disease, less severe than clade Ia but it can lead to severe disease especially in immunocompromised and pregnant individuals, with reports of increased hospitalization.

If clade Ib MPXV importation were to occur, the impact on the general population in Canada would be minor, given that onward transmission is expected to be limited outside of dense sexual networks (e.g., gbMSM communities), with further reductions in spread expected given existing vaccination coverage and ongoing public health campaigns.

Event summary

Three unrelated cases of clade Ib were reported among residents of Los Angeles (LA) County without recent history of international travel, indicating that person-to-person transmission is occurring in California, representing the first report of community transmission in the U.S. All previous cases of mpox due to clade Ib MPXV in the U.S. have been associated with international travel to areas where known clade I MPXV was circulating. ^{Footnote 2} While clade Ib MPXV can infect anyone, California health authorities have indicated that community transmission is currently occurring primarily among gbMSM. All three cases were hospitalized and are now recovering. ^{Footnote 3} In addition to the cases in the U.S., countries reporting autochthonous transmission of MPXV clade Ib as of October, include Spain (one), Italy (two), Portugal (one), the Netherlands (one), and Malaysia (one). ^{Footnote 2 Footnote 4 Footnote 5} These reports suggest a change in transmission patterns, as local chains of transmission are now being detected in the European Union/European Economic Area (EU/EEA) for the first time. The European Centre for Disease Prevention and Control (ECDC) released a threat assessment brief to assess the risk of autochthonous transmission of MPXV clade Ib in EU/EEA countries in the context of male-to-male sexual transmission and assed the overall risk as moderate for gbMSM populations and low for the general population. ^{Footnote 5}

There have been two cases of clade Ib MPXV reported in Canada in November 2024 and September 2025 both related to international travel to East Africa and Lebanon, respectively. ^{Footnote 2} Clade I MPXV wastewater signals (subclade investigation ongoing) were detected at two adjacent catchments sites in Canada the week of October 12, 2025, by PHAC's Wastewater Surveillance Unit. Follow-up sampling and testing in the week of October 19, 2025, were negative for clade I by RT-qPCR.

Considerations for pathogens with pandemic potential

MPXV of any clade is not considered to have pandemic potential, in terms of expected rapid spread within the general population, however due to the high risk of transmission associated with prolonged or intimate contact, spread may occur rapidly through dense sexual networks, e.g., gbMSM sexual networks. Human-to-human transmission can also occur through direct contact with infectious lesions, bodily fluids, respiratory particles, or via direct contact with personal items. Medical countermeasures, including pre- and post- exposure vaccines and antivirals are available. Two-dose Imvamune coverage is estimated to be approximately 82% effective against clade II MPXV infection with no evidence or biological rationale to suggest reduced vaccine effectiveness against clade Ib MPXV. However, emerging evidence suggesting waning humoral immunity in vaccinated individuals, combined with uncertainty around its protection against infection, limits confidence that those vaccinated in 2022 and 2023 continue to have immunity conferred by vaccination. There are no well-established treatments for mpox, but some antiviral medications may be considered on a case-by-case basis.

Risk assessment details

Risk component and rationale

Risk component: estimate [Uncertainty]	Rationale
Likelihood of importation leading to at least one locally acquired case in: gbMSM sexual networks: High [moderate] General population: Very low [moderate]	The previous PHAC rapid risk assessment (Sept 2024) assessed the likelihood of importation of mpox cases due to clade Ib MPXV and spread into Canada as high (moderate uncertainty). Footnote 1 In the past two weeks, multiple countries with frequent travel ties to Canada have reported locally acquired cases of mpox due to clade Ib MPXV, confirming community transmission outside Africa, and increasing the likelihood of importation to Canada. Current transmission patterns and past outbreaks outside of Africa indicate that cases are primarily occurring within sexual networks among gbMSM. Footnote 5 Footnote 6 Footnote 7 The likelihood of exposure within this population is higher, based on historical data from the 2022-2023 mpox outbreaks caused by clade IIb MPXV. Footnote 5 As of September 2025, only 41.3% of Ontarians who received a first dose of the Imvamune vaccine had received their second dose. Footnote 8 Monthly average travel volumes in 2023 and 2024 for November and December from most countries recently reporting community transmission of clade Ib MPXV and with Canada as a destination are substantially higher, in some instances approximately 17 to 20 times higher, than those from African countries that previously reported clade Ib MPXV (source: International Air Transport Association (IATA) unpublished data, 2023 and 2024). Temporary increases in travel are expected during the assessment period, due to the Major League Baseball World Series (held in Toronto and LA), and holiday-related travel (e.g., American Thanksgiving, Christmas and New Years), elevating importation likelihood. In Canada, two travel-related cases of clade Ib MPXV have been detected, with no secondary cases identified (Nov 2024, Sept 2025). The week of October 12, 2025, clade I MPXV (subclade investigation ongoing) was detected in two adjacent wastewater catchment sites. However, follow-up testing the week of October 19, was negative for clade I detection and currently no clinical cases have been identified in the area. In summary, these cases and intermittent environmental detections, along with what is observed by the ECDC, suggest possible undetected cases in the community and importation has already occurred. The transmissibility of clade Ib MPXV is not yet fully understood outside of endemic African countries, but expanded geographical distribution and emergence within gbMSM networks increases the likelihood of importation into Canada. Footnote 1 Footnote 4
Individual impact: Minor [moderate]	Emerging evidence suggests that mpox cases of clade Ib MPXV are less severe than clade Ia, but more severe than clade IIb. Typical clinical features of mpox cases of clade Ib MPXV include prodromal symptoms (e.g., fatigue, malaise), active skin and mucosal lesions, and lymphadenopathy. Initial evidence indicates that clinical manifestations may resolve within two to four weeks. A prospective study of hospitalized patients with clade Ib MPXV reported CFRs <1%. Footnote 9 The individual impact may be higher among vulnerable groups at higher risk of severe illness and complications, including young children, pregnant individuals, and immunocompromised individuals, such as those living with untreated HIV. Footnote 9 Among recent cases, all three locally acquired mpox cases of clade Ib MPXV in California required hospitalization and have since recovered, while all European cases (5) with available information, had mild symptoms, and did not require hospitalization. Footnote 5 Footnote 10 Modified vaccinia Ankara-BN vaccine(e.g., Imvamune) has shown 82% effectiveness for individuals receiving two doses, however the duration of protection is currently unknown. Footnote 11 There is currently no published evidence or biological rationale to suggest reduced effectiveness against MPXV clade Ib. Footnote 11 The use of Imvamune as post-exposure prophylaxis is estimated to be approximately 20% effective. Footnote 11
Population level impact: Minor [low]	Onward transmission of imported clade Ib MPXV into Canada is expected to be limited to close household contacts and among non-household close contacts, including sexual contacts. In Europe, all imported cases resulted in limited onward transmission, mainly to household contacts. Footnote 5 To date, wide-spread community transmission of clade I MPXV has not been established outside of Africa. Footnote 5 In Canada, the two imported cases did not result in secondary transmission. Importation into dense sexual networks (e.g., gbMSM communities) may lead to increased spread within the network, though the extent of spread may be attenuated by existing vaccination coverage and ongoing public health campaigns.  While preliminary data suggest mild to moderate clinical severity, increased incidence of cases adds additional strain on resources for case and contact management, health promotion and vaccination. Among immunocompromised individuals, preliminary evidence indicates a higher risk of hospitalization and severe disease, which could be an added burden on the healthcare system.  Wastewater signals of clade I MPXV detected in two catchment sites may indicate importation and/or undetected/mild case(s) in the community. There are ongoing efforts to determine subclade for these detections. The ability to detect, diagnose and respond to mpox cases in Canada is expected to continue, as the National Microbiology Laboratory (NML) has capacity to support clade differentiation and subclade identification for clinical samples submitted by provinces and territories. Supportive care remains central to mpox management. Although data on clinical effectiveness of antiviral treatments (e.g., tecovirimat) are limited, antivirals can be considered for use in severe cases. Footnote 12

Limitations, knowledge gaps, and uncertainties

The overall uncertainty in this assessment is moderate due to limited information on the number of cases to date outside of the African region and gaps in information about the cases in countries with newly identified community transmission.

Specific sources of uncertainty and knowledge gaps include:

• Uncertainty about whether this clade will exhibit similar severity, transmissibility, vaccine efficacy, and treatment efficacy, as previous clades and in non-endemic countries
• Lack of well-established pharmaceuticals for mpox
• Missing epidemiological, genomic, and clinical information on recently reported cases and proportion of cases among gbMSM and other populations
• The proportion of subclinical cases
• Challenges in correlating wastewater signals to cases, especially if cases are sub-clinical or do not present to healthcare facilities

The main drivers that would increase the risk are: a case detected in Canada with no related travel, if there continue to be wastewater signals in Canada or expanded geographic detection, evidence of expanded transmission beyond close contacts or high number of contacts who acquire the infection, cases in those under 15 years of age (as seen in the DRC) and increased clusters of local transmission in countries with high travel ties.

Proposed actions

Recommendations provided below are based on findings of this risk assessment and include current efforts by local, provincial, and federal public health partners. Additional strategies are included for consideration by jurisdictions according to their local epidemiology, policies, resources, and priorities:

• Review currently available material to support preparedness/response (e.g. guidance, response plans); propose new material/updates if needed
• Consider targeted messaging and outreach to at risk populations and to clinicians regarding prevention, including pre- and post-exposure vaccination, identification and management
• Consider enhanced laboratory capacity for conducting clade differentiation of clinical samples and wastewater samples that are positive for MPXV
• Continue information sharing with federal, provincial/territorial, and local partners on new clade Ib MPXV cases to inform on Canada’s preparedness and response.

Reassessment

The risk assessment team will reconvene to review new evidence and evaluate the need for reassessment if the situation escalates and cases are detected in Canada with no travel history identified, or an importation occurs leading to subsequent clusters of cases.

Methods

The rapid risk assessment (RRA) methodology is based on the interim World Health Organization (WHO) Member State RRA tool. ^{Footnote 13} Likelihood, impact, and overall risk were estimated using previously described scales and risk matrix (see risk assessment methods page, and capacity to respond was estimated using the WHO RRA 2012 guidelines. ^{Footnote 14}

Acknowledgements

Completed by the Public Health Agency of Canada's Centre for Applied Public Health Science Directorate within the Science and Policy Integration Branch.

Risk Assessment Hub members:

Rukshanda Ahmad, Sandra Radons Arneson, Dima Ayache, Raquel Farias, Julia Mielczarek, Linda Vrbova

Lead and supporting programs:

IDVPB-CCDIC-STBBI-SD: Julia Paul, Marianne Stefopulos, Stephanie Totten
ROEMB-CBTH-OTH: Rhea Ferguson, Emma Wilson-Pease
NMLB-SRS-BPAW-WW: Chrystal Landgraff, Chand Mangat, Edgard Mejia
IDVPB-CISP-NACI: Nicole Forbes, Joshua Montroy

Subject Matter Experts:

Mable Chan, Andrea Chittle, Maryam Kamkar, Tiffany Locke