A recombinant mpox virus turned up twice, and the routine test called it two clades

The World Health Organization reported on 14 February that two people have been infected with a monkeypox virus carrying genomic elements of both clade Ib and clade IIb — the first documented inter-clade recombinant strain of MPXV [s1].

Neither patient experienced severe outcomes, both had a clinical presentation similar to that seen with other clades, contact tracing was completed in both reporting countries, and no secondary cases were detected [s1]. WHO's overall public health risk assessment for mpox is unchanged: moderate for men who have sex with men with new or multiple partners and for sex workers or others with multiple casual sexual partners, and low for the general population without specific risk factors [s1].

So the immediate clinical story is unremarkable. The surveillance story is not.

How the two cases were found

The first reported detection came from the United Kingdom in December 2025 [s1]. The virus was identified from a vesicular swab taken from a traveller who had returned from a country in the Asia Pacific region in October 2025 [s1]. On initial laboratory confirmation, clade differentiation qPCR typed it as clade Ib [s1].

Whole genome sequencing then showed something else: a genome with regions resembling both clade Ib and clade IIb reference sequences [s1]. Because that is an unusual result, sequencing was repeated on the original extract, a fresh extract from the same primary sample, a second swab collected from the patient at the same time, and a cultured isolate [s1]. All produced identical genome sequences — confirming the recombinant and also showing that it replicates and has potential for onward transmission [s1].

The second case came to light in reverse. On 13 January 2026 India's National IHR Focal Point notified WHO of an inter-clade recombinant found in samples from a man who had presented for care in September 2025 [s1]. He had recently travelled from a country in the Arabian Peninsula, where he works [s1]. He developed symptoms on 1 September while still abroad, PCR confirmed MPXV on 11 September after his return, and clade differentiation PCR on 15 September identified the virus as clade II [s1]. Initial sequencing suggested clade IIb [s1].

The reclassification followed an update to the global Nextclade database on 16 December 2025, which added the recombinant strain reported by the United Kingdom [s1]. Against that reference, the Indian sample was reassigned [s1]. The patient was hospitalised, had no medical complications, recovered fully, and tested negative on 29 September 2025 [s1]. He reported no close contacts in India, and no secondary cases were identified [s1].

The finding buried in the methods

Read the two case descriptions side by side and one detail stands out. The UK case typed as clade Ib by clade differentiation PCR. The India case typed as clade II by the same class of assay [s1].

Same virus. Two different answers.

WHO states the implication directly: clade differentiation PCR assays alone may not reliably identify recombinant MPXV strains, and genomic sequencing is likely to be required to detect them [s1].

That is a meaningful constraint. Clade differentiation PCR is fast, cheap and widely deployed; whole genome sequencing is none of those things. A recombinant that can present as either parent clade depending on which region the assay happens to target is, in effect, invisible to the tier of testing most laboratories run first.

What the genomes suggest about spread

Full or near-full genome retrieval — more than 99% — from both the Indian sample and a sample-derived isolate enabled phylogenetic analysis showing greater than 99.9% similarity to the UK strain [s1]. Thirty-four recombinant tracts were observed in the sequence reported by India and 28 in the sequence reported by the United Kingdom, with 16 tracts common to both [s1].

Symptom onset in the Indian case preceded the UK case by more than two months, making India's the earliest known detection of this recombinant globally [s1]. WHO draws two conclusions: the origin of the recombinant strain remains unknown, and transmission already involves at least four countries across three WHO regions, so it is likely to be more widespread than currently documented [s1].

Four countries because each patient acquired infection elsewhere — one linked to travel from a country in South-East Asia, one to a country in the Arabian Peninsula — and neither source country has reported the strain [s1].

What recombination is, and is not

Recombination is a known natural process that occurs when two related viruses infecting the same individual exchange genetic material, producing a new virus [s1]. WHO notes that multiple MPXV strains are circulating through interconnected sexual networks across many countries, so co-infection with different strains, and therefore emergence of recombinants, can be expected even if it is rare [s1].

The agency is explicit about the limits of what two cases can support: with so few cases found to date, conclusions about transmissibility or clinical characterisation of mpox due to recombinant strains would be premature [s1]. UK authorities are continuing phenotypic characterisation studies [s1].

WHO advises all countries to remain alert to the possibility of MPXV genetic recombination, and says the public health risk of any newly detected recombinant should be assessed case by case using epidemiological, clinical and genomic information [s1].

What to watch

Whether further cases of this strain surface, particularly in the two source countries implied by the travel histories. The two-month gap between the earliest and latest known case, combined with near identity of the genomes, is the reason WHO expects there are more.

Whether laboratories that rely on clade differentiation PCR adjust their sequencing triggers. Detection of the next recombinant depends on someone sequencing a sample that the fast test has already answered.

And whether phenotypic work in the UK produces any evidence bearing on transmissibility or severity. Until it does, the honest position is that nothing is known about how this virus behaves relative to its parents.

Sources

• [s1] World Health Organization, "Disease Outbreak News: Mpox – recombinant virus with genomic elements of clades Ib and IIb – Global situation," 14 February 2026. https://www.who.int/emergencies/disease-outbreak-news/item/2026-DON595