2024 Mpox Clade Ib Outbreak

The emergence of Clade Ib mpox in the DRC and its spread to East Africa triggered the seventh WHO PHEIC in history in August 2024.

Outbreak Summary (as of May 2025)

What Makes Clade Ib Different

Clade I mpox has circulated endemically in the Congo Basin for decades, transmitted primarily from animal reservoirs and through household contact with a CFR of 3–11%. Clade Ib is a new sub-lineage that emerged in South Kivu's mining communities in 2023–2024, spreading efficiently through sexual contact networks — similar to how Clade IIb spread globally in 2022, but with Clade I's higher baseline severity. Genomic analysis shows Clade Ib has accumulated mutations that facilitate human-to-human transmission while retaining Clade I virulence. The combination of increased transmissibility via sexual networks and higher severity distinguishes Clade Ib from both prior Clade I (less efficiently sexually transmitted) and Clade IIb (less severe).

Response and Vaccination

International vaccine donations of JYNNEOS reached the DRC following the PHEIC declaration. Africa CDC coordinated continent-wide response under its new Public Health Emergency of Continental Security (PHECS) framework — the first use of this new mechanism. Key challenges: vaccine delivery to remote South Kivu communities; conflict-affected areas; cold chain logistics; and the need for community engagement in populations with distrust of health authorities. European countries updated travel guidance for visitors from/to affected African countries. No evidence of sustained Clade Ib community transmission in Europe or North America as of May 2025, though travel-related imports have occurred.

Sources: WHO mpox Clade Ib situation reports; Africa CDC PHECS declaration; Nature Medicine (Clade Ib genomics); ECDC rapid risk assessment Clade Ib 2024.

Related: Mpox overview · 2022 global mpox · DRC mpox

The DRC's Longstanding Mpox Burden Before Clade Ib

To understand Clade Ib, context about the DRC's longstanding mpox epidemic is essential. Mpox has been endemic in the Congo Basin since at least the 1970s, historically considered a zoonotic disease transmitted from rodents to humans with limited human-to-human spread. The DRC consistently reported thousands of suspected cases annually, but global attention remained minimal. The 2022 global Clade IIb outbreak dramatically changed international awareness of mpox — and in its wake, increased surveillance revealed that the DRC's Clade I endemic was far larger than previously recognized: over 14,000 suspected cases in 2022, rising in subsequent years. Clade Ib emerged from within this already severely burdened epidemic, in the mining communities of South Kivu where sexual contact networks provided efficient transmission chains not previously associated with Clade I mpox. The emergence was not a sudden event but the product of conditions building over years.

Spread to East Africa and International Cases

Clade Ib's spread from South Kivu, DRC into neighboring East African countries followed predictable cross-border population movement routes, including sex workers and truck drivers transiting the porous borders of the Great Lakes region. Rwanda confirmed Clade Ib cases in September 2024 — the first time mpox had been confirmed in Rwanda's history. Burundi, Uganda, and Kenya followed in subsequent weeks. The pattern raised alarm because East Africa had no prior mpox immunity (historically a Congo Basin disease), and JYNNEOS vaccine supply for the region was extremely limited at the time of the PHEIC declaration. Imported cases to Europe — in Sweden, Germany, and the UK — were travel-related and no sustained community transmission was detected in high-income countries as of mid-2025. But the speed of geographic spread within East Africa demonstrated that Clade Ib was not geographically confined to its origin zone in South Kivu.

Open Questions for 2025 and Beyond

As of mid-2025, several critical questions shape Clade Ib's trajectory. First: will effective vaccination coverage reach DRC in time to interrupt transmission, given the logistical challenges of cold-chain vaccine delivery in active conflict zones? Second: is sexual network transmission in mining communities the primary driver, or does household transmission play a larger role than understood? The relative contribution shapes whether condom promotion or ring vaccination is the more effective intervention priority. Third: will Clade Ib follow the 2022 Clade IIb trajectory of spread through international sexual networks into high-income countries? WHO's 2025 assessment was that Clade Ib had not established self-sustaining transmission outside Africa — but the risk of amplification through international travel networks remains a subject of ongoing surveillance and modeling. The PHEIC declaration remains an important lever for maintaining political and financial attention on a crisis occurring primarily in one of the world's poorest regions.