The Lancet (Correspondence): Clade IIb Mpox Outbreak in Sierra Leone

 

#18,766

Ten days ago, in  Preprint & Update On Sierra Leone 2025 Mpox IIb Outbreak, we looked at growing concerns over a large outbreak of a new strain of Mpox (clade IIb) which has recently emerged in the West African nation of Sierra Leone. 

We know of at least 4 clades of Mpox(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.

But within each clade there can be multiple strains, which may affect the virus's behavior. While the more aggressive Clade Ib virus still commands most of our attention, recent reports suggest this new mpox clade IIb strain reported by Sierra Leone behaves in an unexpectedly aggressive manner. 

The latest  epi trends chart , from the WHO's May 29th Mpox Epidemiological Report shows this sudden rise in cases in Sierra Leone (orange) over the past few months. 

Last month Virological.org posted an analysis of this emerging Mpox Clade IIb variant (G.1) in Genomic epidemiology of Mpox virus in Sierra Leone, which they describe as likely doubling cases every two weeks, and having a reproductive number (R0) of > 2.

On Friday The Lancet published a correspondence (link below) which reports on unpublished genomic sequencing of 77 samples which further identifies this new clade IIb lineage as A.2.2.1; a genetically distinct offshoot from the Nigerian A.2.2 lineage.

As of June 4, 2025, Sierra Leone reports 3,682 confirmed cases, although the true number likely far higher. Freetown accounts for roughly 60% of the cases, but surveillance and testing outside of the Capital city is far less comprehensive. 

Since late April > 600 new cases have been reported each week. 

Unusually, the percentage of females infected (47.5%) is nearly equal to that of males (52.5%). The international spread of clade IIb which began in 2022 has primarily affected males (90%+) (cite MMWR), most of whom identified as gay or bisexual. 

The authors note that the demographics and spread of this new mpox clade IIb virus more closely resembles larger Clade Ib outbreaks in countries like the DRC, Burundi, and Uganda. 

Follow the link to read the full correspondence.  I'll have a postscript after the break. 

Clade IIb mpox outbreak in Sierra Leone
Oriol Mitjàa,b ∙ Deborah Watson-Jonesc,d ∙ Edward M Choic ∙ Mohamed Boi Jallohe,f ∙ Foday Sahre

Online first June 20, 2025

According to this report, an Mpox vaccination campaign began in March, and to date > 43,000 (of roughly 80,000 doses) have been administered.  But with a population of 8.7 million people, the current supply of vaccine is insufficient for a large-scale vaccination campaign. 

A 400-bed Mpox hospital has been set up in Freetown, but once again - with 600+ new cases a week - many cases are forced to isolate at home (albeit, some chose to do so due to personal preference). 

There are genuine concerns that this currently localized outbreak may spread into neighboring countries (Liberia, Togo and Ghana) - or worse - began a world tour like an earlier clade IIb virus did in 2022. 

And of course, we are still in a PHEIC (Public Health Emergency of International Concern) declared over the rapid spread of Mpox clade Ib (see map below).

Since the eradication of smallpox in the 1970s, there has been a general feeling that poxviruses are a thing of the past, a relic of the 20th century.  Today, few remember that in the 20th century alone, smallpox claimed upwards of 500 million lives. 

While Mpox is considerably less deadly, it continues to evolve - both in reservoir animal hosts and in humans - and the emergence of new, potentially more problematic clades/lineages, are always possible.

But even at today's severity, the international spread of yet another Mpox virus would constitute a major public health crisis. One which we should be trying both to avert, and working to prepare for.