Multiple introductions of monkeypox virus to Ireland during the international mpox outbreak, May 2022 to October 2023

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Gabriel Gonzalez1,2,3 , Michael Carr1,3 , Tomás M Kelleher¹ , Emer O’Byrne¹ , Weronika Banka¹ , Brian Keogan¹ , Charlene Bennett¹ , Geraldine Franzoni¹ , Patrice Keane⁴ , Cliona Kenna¹ , Luke W Meredith⁵ , Nicola Fletcher6,7 , Jose Maria Urtasun-Elizari¹ , Jonathan Dean¹ , Ciaran Browne⁸ , Fiona Lyons⁹ , Brendan Crowley⁴ , Derval Igoe10 , Eve Robinson11 , Greg Martin11 , Jeff Connell¹ , Cillian F De Gascun¹ , Daniel Hare¹ 1. UCD National Virus Reference Laboratory, University College Dublin, Dublin, Ireland 2. Japan Initiative for World-leading Vaccine Research and Development Centers, Hokkaido University, Institute for Vaccine Research and Development, Sapporo, Japan 3. International Collaboration Unit, International Institute for Zoonosis Control, Hokkaido University, Sapporo, Japan 4. Department of Virology, St. James’s Hospital, Dublin, Ireland 5. Department of Pathology, University of Cambridge, Cambridge, United Kingdom 6. Veterinary Sciences Centre, University College Dublin, Dublin, Ireland 7. Centre for Experimental Pathogen Host Research, University College Dublin, Dublin, Ireland 8. National MPOX Crisis Management Lead, Acute Operations, Health Service Executive, Dublin, Ireland 9. Sexual Health and Crisis Pregnancy Programme, Health and Wellbeing, Strategy and Research, Healthcare Strategy, Health Service Executive, Dublin, Ireland 10. Health Service Executive Public Health: National Health Protection, Ireland 11. Health Protection Surveillance Centre, Dublin, Ireland Correspondence: Daniel Hare (
Euro Surveill. 2024;29(16):pii=2300505.

Background: Mpox, caused by monkeypox virus (MPXV), was considered a rare zoonotic disease
before May 2022, when a global epidemic of cases in non-endemic countries led to the declaration of a
Public Health Emergency of International Concern.
Cases of mpox in Ireland, a country without previous mpox reports, could reflect extended local transmission
or multiple epidemiological introductions.

Aim: To elucidate the origins and molecular characteristics of MPXV circulating in Ireland between May 2022 and
October 2023. Methods: Whole genome sequencing of MPXV from 75% of all Irish mpox cases (182/242) was
performed and compared to sequences retrieved from public databases (n = 3,362). Bayesian approaches
were used to infer divergence time between sequence from different subclades and evaluate putative importation
events from other countries.

Results: Of 242 detected mpox cases, 99% were males (median age: 35
years; range: 15–60). All 182 analysed genomes were assigned to Clade IIb and, presence of 12 distinguishable
subclades suggests multiple introductions into Ireland. Estimation of time to divergence of subclades
further supports the hypothesis for multiple importation events from numerous countries, indicative of
extended and sustained international spread of mpox. Further analysis of sequences revealed that 92% of
nucleotide mutations were from cytosine to thymine (or from guanine to adenine), leading to a high number
of non-synonymous mutations across subclades; mutations associated with tecovirimat resistance
were not observed.

Conclusion: We provide insights into the international transmission dynamics supporting
multiple introductions of MPXV into Ireland. Such information supported the implementation of
evidence-informed public health control measures.