Geographic Range of Monkeypox in 2021 - Credit WHO
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On Monday, the UK's HSA announced a New Recombinant Strain of Mpox Virus identified in a traveler recently returned from Asia - one that carried elements of both clade Ib and clade IIb of the virus (see Virological.org Inter-Clade Recombinant Mpox Virus Detected in England in a Traveller Recently Returned from Asia).While epidemiological details are scant, this is the 3rd new mpox variant to emerge in the past 4 years (Ib and IIb emerged in 2022 and in 2023, respectively), and it seems likely that more may emerge over time.
All of this was predicted more than a decade ago in a 2014 EID Journal article Genomic Variability of Monkeypox Virus among Humans, Democratic Republic of the Congo, where the authors cautioned:
Small genetic changes could favor adaptation to a human host, and this potential is greatest for pathogens with moderate transmission rates (such as MPXV) (40). The ability to spread rapidly and efficiently from human to human could enhance spread by travelers to new regions.
Four years ago, mpox (formerly `Monkeypox') was typically only found in central and west Africa, with just a handful of cases exported each year (see WHO graphic at top of blog).
In the spring of 2022, Mpox clade IIb emerged, and began its world tour. According to the CDC; The ongoing global outbreak of clade IIb monkeypox has caused more than 100,000 cases in 122 total countries, including 115 countries where monkeypox was not previously reported.
While that outbreak eventually slowed, it has never really ended - and in the summer of 2024 the WHO declared a new emerging threat; Clade Ib (see WHO declared the Mpox outbreak in Central Africa to constitute a PHEIC ).
Since then the global picture has changed radically with the latest WHO Epidemiological Update (Dec 8th) reporting `In October 2025, 44 countries, across all WHO regions, reported a total of 2501 new confirmed mpox cases, including 12 deaths (case fatality ratio [CFR] 0.5%).'
The significance and potential impact of the UK's newly reported recombinant Mpox case remain unknown, but the pattern we've seen over the past decade is - given half a chance - even obscure poxviruses can grow to become international threats.
Dr Jonas Albarnaz, Institute Fellow, Capripoxvirus Biology, Pirbright Institute, said:
“The detection of a new recombinant mpox in the UK with genetic elements of both clade IIb and clade Ib is concerning because this shows that the continued global circulation of mpox since 2022 is enabling the virus to evolve in the human population. Clade IIb virus was responsible for the international mpox outbreak in 2022 that subsided in Western countries but did not stop to circulate entirely.
Clade Ib emerged in the Democratic Republic of Congo (DRC) in 2023 and quickly spread to other neighbouring countries in Africa, and spread outside Africa in 2024, via international travel including to the UK. There was no evidence that clade Ib established community circulation outside Africa. This changed recently with the identification of clade Ib mpox in the US, Spain, Italy, the Netherlands, and Portugal in individuals without travel history, suggesting that clade Ib also circulates locally, albeit in small numbers, in these countries.
“Both clade Ib and IIb have patterns of mutations that indicate these viruses evolved during sustained human-to-human transmission. The identification of a new recombinant between Ib and IIb shows recombination might also contribute to mpox virus evolution. Recombination occurs when two related viruses infect the same cell (‘co-infection’) and exchange bits of their genetic material, generating a recombinant that combines properties of both co-infecting viruses. Co-infections are rare, but poxviruses, the group that include mpox, are known to be prone to recombination. If community transmission ensues, co-circulation of clade Ib and IIb will increase the chances of co-infections with both clades, which is a pre-requisite for recombination.
“From just one case it is impossible to gather much information about this new recombinant. However, its detection highlights the importance of continued genomic surveillance to detect new mpox variants early on. Clade I virus is associated with a more severe disease and higher mortality rates than the clade II virus, but we do not know if this recombinant transmits or cause disease differently either Ib or IIb.
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Dr Boghuma Titanji, Assistant Professor of Medicine, Emory University, said:
“The identification of a recombinant mpox strain containing elements of both Clade I and Clade II is precisely what experts in the field feared would happen if the virus continued to spread globally without a decisive response to stop it. Orthopoxviruses are well known for their ability to exchange portions of their genome and recombine to generate new variants, this is a core mechanism of their evolution. The key concern now is whether events like this will alter the virus’s transmissibility or virulence. There are also implications for how well existing testing platforms can identify these emerging recombinant strains. The more mpox circulation we permit, the more opportunities the virus has to recombine and adapt, further entrenching mpox virus as a human pathogen that is not going away.”
Continue on to the Science Media Center website to read additional insights from:
But history is made by outliers, and they come along often enough that we can't afford to ignore burgeoning threats.
Stay tuned.
