#18,754
In 2022 Clade IIb emerged on the international stage, after simmering in West and Central Africa for decades. While the first Public Health Emergency of International Concern (PHEIC) was rescinded in 2023, a second - more aggressive Mpox virus (Clade Ib) - began spreading globally in 2024.
Over the past 4 years we've seen Mpox expand from a handful of Central African nations to encompass much of the world.
We now have at least 4 clades of Mpox in circulation (I, Ib, II, IIb), with Clade Ia and Ib considered the most severe. Two clades (IIb and, to a lesser extent Ib) have managed to spread internationally, and the WHO's most recent risk assessment puts their global risk as Moderate to High.
While the more aggressive Clade Ib virus still commands most of our attention, since the first of the year we've seen a steep rise in clade IIb cases in Sierra Leone, and reports that a new, reportedly more aggressive, variant has emerged.
The epi trends chart below, from the WHO's latest Mpox Epidemiological Report shows this sudden rise in cases in Sierra Leone (orange) over the past few months.
Although data is scarce, this would represent a much faster growth rate than we've seen with older clade IIb variants. And while Clade IIb spread has often been associated with the MSM community, G.1 appears to be spreading widely across all genders, age groups, and geographies within Sierra Leone.
Over the past couple of weeks the world has taken notice, with reports such as:
Rapid transmission through sexual networks raises fears of wider spread in the region
2 Jun 2025 By Kai Kupferschmidt
Earlier this week, a preprint was published on medRxiv describing this recent `historic' increase in Mpox in Sierra Leone. It is, admittedly, based on a very small subset of cases (n=187) out of what reportedly may already exceed 10,000 cases.
Jia B Kangbai, Emmanuel Saidu, Ibrahim K. Foday, Emmanuel S. Kamanda, Michaella Jaba, Allan Campbell, Alie Brima Tia, Christopher M Ruis, Lorenzo Subissi, Megan HalbrookSydney Merritt, Nicole A Hoff, Eddy Kinganda-Lusamaki, Laurens Liesenborghs, Isaac I Bogoch, Souradet Y Shaw, Placide Mbala-Kingebeni, Anne W Rimoin, Jason Kindrachuk
doi: https://doi.org/10.1101/2025.05.30.25328691
Preview PDF
ABSTRACT
We analyzed available mpox cases reports in Sierra Leone from January to June 2025 following a historic increase in mpox cases across multiple urban and rural regions in the country.Here, we assessed demographic and clinical data from 187 case reports. Mpox case reports in this study were primarily from Bo (86/187; 46.0%) and Freetown (48/124; 25.7%). Most case reports were from male patients (101/187; 54.0%) as compared to females (86/187; 46.0%), with an overall median age of 26 years (IQR 22-34). The median age was higher among males (30 years; IQR 25-39) then females (24 years; IQR 21-27).Rash, fever, headache, lesions, and generalized pain were reported for all patients, where data was reported. Lymphadenopathy, muscle pain, sore throat, and cough were reported less commonly. Age- and sex-specific interventions, as well as community engagement that includes historically stigmatized groups are critical for mpox containment and mitigation measures in Sierra Leone.
(SNIP)
This report highlights early clinical and epidemiological observations from the rapidly expanding mpox epidemic currently ongoing in Sierra Leone. Mpox cases totals in 2025 have vastly surpassed the cumulative total of all mpox cases in-country from 1970 through 2024 with the 3782 cumulative cases surpassing those of Nigeria following the re-emergence of mpox in 2017 through 2024 (3,771 suspected mpox cases) [26]. Containment and mitigation efforts, including vaccine deployment to frontline healthcare workers and those at increased risk for infection, as well as increased community engagement to at-risk populations and increased access to testing (and diagnostic infrastructure) are needed.
While DNA viruses (like Mpox) often mutate at a slower rate than RNA viruses (like influenza or SARS-CoV-2), they are far from static, and longer chains of infection (or infecting different host species) can help promote faster evolution (see Evolution of monkeypox virus from 2017 to 2022: In the light of point mutations).
Another prescient study - published just 3 months before the 2022 international outbreak of Mpox Clade II - warned of the potential for Monkeypox to spread (see PLoS NTD: The Changing Epidemiology of Human Monkeypox—A potential threat?).
In 2023, in Mpox Virus: Its Molecular Evolution and Potential Impact on Viral Epidemiology by Xi Yu et al., the authors wrote:
The evolution of zoonotic infections to become more transmissible or virulent in humans is a key cause for concern. Particularly, regarding OPXVs such as MPXV, there is concern about the risk that they could evolve into infections capable of causing another smallpox-like pandemic.
One of the realities of life in this third decade of the 21st century is that the world is a lot smaller than used to be. Vast oceans and extended travel times no longer offer the protection they once did, and there is no technological shield that we can erect that would keep an emerging virus out.
Whether this surge of Mpox in Sierra Leone is a flash in the pan, or the start of something bigger, remains to be seen. Surveillance, testing, and reporting isn't as robust as we'd like, and these poxviruses continue to evolve in unpredictable ways.
But it is a reminder that Nature's laboratory is open 24/7, and each day brings us a little closer to the next global public health crisis.