A newly identified strain of the mpox virus, known as Clade Ib, has put the infectious disease back in the headlines in the United Kingdom. While health authorities maintain that the overall risk to the general public is currently low, this development has raised fresh concerns due to the historically more severe nature of viruses from the Clade I family.
Understanding the Threat: Clade I vs. Clade II
Mpox, formerly called monkeypox, is related to the smallpox virus but is typically much milder. The virus is divided into different genetic groups called clades. The global outbreak in 2022 was driven by Clade II, which tends to cause less severe illness and has a fatality rate of well under 1%.
The newly highlighted Clade Ib is a subset of Clade I. Clade I viruses have historically circulated in Central Africa and are associated with more serious disease. A recent review of outbreaks from 2022-2025 notes that Clade I infections have had case fatality rates between 3% and 10% in African settings with limited healthcare access. Exact death rates for the specific Ib lineage are still under study, but it falls under this more severe family.
Symptoms and Current UK Situation
Clinically, Clade Ib infections present similarly to other mpox cases. Symptoms usually begin 5 to 21 days after infection and include:
- Fever, chills, headache, muscle and back aches.
- Swollen lymph nodes in the neck, armpits, or groin.
- Pronounced fatigue and low energy.
This is followed by a rash that progresses from flat spots to raised bumps, then blisters, and finally scabs. The rash can affect the face, limbs, palms, soles, mouth, genitals, and anus. Recent UK reports emphasize that many current patients have a limited number of painful lesions focused on the genital and anal areas, sometimes accompanied by proctitis (rectal inflammation) causing significant pain and bleeding.
In December 2025, the UK Health Security Agency (UKHSA) confirmed the identification of this new Clade I-associated strain in England, linked to travel rather than sustained community spread. Crucially, no mpox-related deaths have been reported in the UK in this recent period. Globally, the WHO estimates over 100,000 confirmed cases and several hundred deaths since 2022, mostly in regions where Clade I circulates and healthcare access is limited.
Who is Most at Risk and How the UK is Responding
While most healthy adults recover in 2-4 weeks with supportive care, certain groups are at higher risk for severe complications and death. These include:
- Young children and pregnant individuals.
- People with weakened immune systems (e.g., uncontrolled HIV).
- Those with extensive skin disease or very painful anogenital lesions.
Potential complications can include severe bacterial skin infections, pneumonia, vision-threatening eye involvement, and sepsis.
The UK's response involves heightened surveillance, rapid testing, isolation, and contact tracing for any suspected Clade I case. Targeted vaccination is also offered to those at highest risk. The NHS advises anyone with a new, unexplained rash or blisters—especially on the genitals or anus—alongside fever or swollen glands to seek medical advice promptly.
For the vast majority in the UK, the risk from Clade Ib mpox remains low. The key public health messages are to recognize the symptoms, seek early medical advice if exposed, and follow vaccination guidance if eligible.
