Mpox Clade Ib Outbreak: WHO Emergency Declaration and Global Update 2026

What Is Mpox Clade Ib and How Does It Differ from Clade IIb?

Monkeypox virus (MPXV) is divided into two major clades: clade I (formerly the Congo Basin clade) and clade II (formerly the West African clade). Historically, clade I has been associated with higher virulence and case fatality rates (up to 10% in some outbreaks, and higher in children and immunocompromised individuals), while clade II has been milder (case fatality rate below 1%). The 2022 global outbreak that spread to over 110 countries was caused by clade IIb and primarily affected men who have sex with men (MSM), with sexual contact as the main transmission route.

The current outbreak involves clade Ib, a newly identified sublineage of clade I that was first characterized in September 2023 in South Kivu province, DRC. Genomic analysis indicates that clade Ib has acquired mutations that may facilitate more efficient human-to-human transmission compared to historical clade I strains, which were primarily zoonotic (transmitted from animals to humans). Clade Ib has been spreading predominantly through sexual contact and close physical contact, including in sex worker networks and among displaced populations in conflict-affected areas of eastern DRC.

Clinically, clade Ib infection tends to present with a more extensive rash that is often concentrated in the genital and perianal regions (consistent with sexual transmission), though widespread disseminated rash can also occur. The case fatality rate for clade Ib appears to be approximately 3-5%, significantly higher than clade IIb (approximately 0.1-0.2% in high-income countries during 2022) but lower than historical clade I outbreaks, possibly due to the younger, less immunocompromised population affected. Children under 15 account for a disproportionate share of cases and deaths in the DRC, reflecting both household transmission and higher vulnerability in pediatric populations.

Why Did the WHO Declare a Public Health Emergency?

The WHO Director-General, Dr. Tedros Adhanom Ghebreyesus, declared mpox a Public Health Emergency of International Concern (PHEIC) on August 14, 2024, following the recommendation of the International Health Regulations (IHR) Emergency Committee. This was the second mpox PHEIC declaration — the first was issued during the 2022 clade IIb global outbreak and was lifted in May 2023. The 2024 declaration was triggered by the alarming increase in clade I mpox cases in the DRC (which reported over 30,000 suspected cases and more than 900 deaths in 2024) and, critically, the spread of clade Ib to neighboring countries including Burundi, Rwanda, Uganda, and Kenya.

The cross-border spread of clade Ib was particularly concerning because it demonstrated the potential for international dissemination through travel and population movement in a region with significant cross-border mobility, displaced populations, and limited healthcare infrastructure. Cases linked to clade Ib were also reported in travelers returning to countries outside Africa, including Sweden, Thailand, Pakistan, and others, confirming that the virus had the potential for global spread similar to the 2022 outbreak but with a more virulent strain.

The PHEIC declaration serves multiple purposes: it signals the severity of the threat to the global community, triggers WHO temporary recommendations for member states (regarding surveillance, laboratory testing, vaccination, travel measures, and risk communication), and catalyzes international funding and resource mobilization. However, the declaration also highlighted significant global health inequities — during the 2022 outbreak, high-income countries rapidly deployed vaccines and antivirals, while African countries where mpox has been endemic for decades had minimal access to these tools. Addressing this disparity has been a central focus of the 2024 response.

What Vaccines and Treatments Are Available for Mpox?

Jynneos (known as Imvanex in Europe and Imvamune in Canada), manufactured by Bavarian Nordic, is a non-replicating modified vaccinia Ankara (MVA-BN) vaccine that is FDA-approved for prevention of smallpox and mpox in adults 18 and older. The standard regimen is two subcutaneous injections given 28 days apart. During the 2022 outbreak, emergency intradermal administration (one-fifth of the subcutaneous dose) was authorized to extend limited supply, with immunogenicity data supporting this approach. Real-world effectiveness studies from the 2022 outbreak estimated two-dose vaccine effectiveness at approximately 66-86% against mpox infection.

International deployment of Jynneos/Imvanex to the DRC and neighboring countries began in September 2024, facilitated by donations from the US, EU, and Japan, and coordinated through Gavi and the Africa CDC. However, challenges have been substantial: cold chain requirements (the vaccine must be stored at -20°C for long-term storage), limited manufacturing capacity, regulatory barriers (the vaccine was not initially approved for use in the DRC), and the logistical complexity of reaching affected populations in conflict zones. The DRC also received doses of LC16m8, a replicating smallpox vaccine produced in Japan, which has been used in children. A fractional-dose intradermal strategy has been adopted to maximize coverage from limited supply.

Tecovirimat (TPOXX), an antiviral drug originally developed for smallpox, has been the primary treatment for mpox under emergency use protocols. However, the PALM007 randomized controlled trial conducted in the DRC — the first large RCT of tecovirimat for mpox — reported in 2024 that tecovirimat did not significantly reduce the time to lesion resolution compared to placebo (median 28 days vs. 27 days). These results were sobering and highlighted the need for additional therapeutic development. Cidofovir and brincidofovir are alternative antivirals with activity against orthopoxviruses, though their use is limited by toxicity (nephrotoxicity for cidofovir) and limited availability. Supportive care — including pain management, wound care, and treatment of secondary bacterial infections — remains the cornerstone of clinical management.