First major Clade IIb mpox outbreak in Africa: Narrative review of Sierra Leone's 2025 epidemic.

Abstract

The mpox outbreaks observed in 2025 highlighted significant shifts in transmission dynamics, viral clades, and public health responses, particularly in Africa. In contrast, Sierra Leone experienced its largest mpox outbreak that year-Africa's first major Clade IIb epidemic-which ran from 10 January to 17 December 2025. It involved 5,442 confirmed cases among 7,110 suspected cases, a case fatality rate of 1.1%, and 22,536 contacts identified. A public health emergency was declared on 13 January 2025. Transmission peaked in epidemiological week 20 (May 2025) with 654 confirmed cases. The burden was highest in Western Area Urban (55.4%, 3,014 cases), Western Area Rural (19.9%), and Bombali (6.1%). Cases predominantly affected young adults aged 20-39 years (median age 26) and males (52.5%). Key affected groups included sex workers, children under 15 years (6.5%), and pregnant women (1.6%). High population density in Freetown, limited resources, weak coordination, misinformation, and surveillance gaps facilitated spread. Genomic evidence revealed undetected Clade IIb (hMPXV-1 lineage, including G.1) circulating for ~3 months from Nigeria. Environmental contamination affected 13.5% of healthcare surfaces. Symptoms included rash, fever, headache, lymphadenopathy, sore throat, and pain; 28.9% of cases were hospitalized, often delayed by stigma and fear. Among those screened for HIV co-infection (23.6%), 82% of deaths occurred in co-infected individuals. The National Public Health Agency (NPHA) implemented seven response pillars: case management and isolation, infection prevention and control (IPC), risk communication, targeted vaccination (186,470 doses administered from 267,000 received), coordination, surveillance, and research. The outbreak ended after 42 consecutive case-free days, underscoring the importance of early surveillance and genomic monitoring to prevent sustained urban transmission.