New Mpox Variant, Clade Ib, Spreads in US and Europe, Raising Public Health Concerns

A newer variant of mpox, known as clade Ib, is now actively spreading through communities in the United States and Europe, prompting renewed vigilance from health officials. Cases recently detected in California show no apparent links to each other or recent international travel, signaling local transmission, as reported by The Guardian on November 1, 2025. The World Health Organization (WHO) has issued warnings regarding the rising cases and the global spread of this particular strain.

The global mpox situation in 2025 has seen a concerning surge, with more than 44,000 cases reported worldwide, already surpassing the total for 2024, according to the original article. In September 2025 alone, the WHO recorded over 3,100 confirmed cases and 12 deaths globally, with a significant majority, over 80%, originating from the African Region.

Despite the escalating case numbers, the immediate risk to the general public remains low, as stated by public health officials. However, the emergence of this new variant presents increased challenges for health authorities in effectively detecting cases and preventing further spread within communities. The European Centre for Disease Prevention and Control (ECDC) is actively urging heightened awareness and the implementation of targeted prevention measures.

Evidence of local transmission of clade Ib has been confirmed in several countries outside Africa, including Italy, Malaysia, the Netherlands, Portugal, Spain, and the United States, according to the WHO's latest situation report. In California, three unlinked cases were identified in October, raising significant alarm among health officials, The Guardian noted.

Further analysis by the WHO indicates that at least five cases of clade Ib mpox have been detected among men who have sex with men (MSM). This marks the first evidence of previously undetected circulation of this strain within this at-risk population since 2022, as reported by vax-before-travel. This suggests ongoing transmission within sexual networks among MSM in both European countries and the Los Angeles area.

The WHO continues to closely monitor the evolution of clade Ib, emphasizing the critical importance of early detection and rapid treatment to prevent larger outbreaks, as highlighted by ETimes. The organization previously declared the mpox outbreak in Africa a public health emergency of international concern in August 2024 due to a significant rise in clade I cases, according to the CDC.

• Background Context and Clade Ib Origin: Mpox, formerly known as monkeypox, is a viral disease with two primary clades: Clade I and Clade II. While the 2022-2023 global outbreak was predominantly caused by Clade IIb, Clade Ib, a distinct offshoot of Clade I, first emerged in the Democratic Republic of Congo (DRC) in 2023. This newer variant is considered more transmissible and potentially more severe than Clade IIb, according to reports from The BMJ and Symptoms of Mpox.

• Global Case Surge and African Impact: Global mpox cases have experienced a notable increase in 2025, with over 44,000 reported, exceeding the total for 2024. The World Health Organization's 59th multi-country situation report, published on October 30, detailed 2,862 confirmed cases and 17 deaths across 17 African nations within a six-week period. Kenya and Liberia are currently witnessing sharp rises in cases, while the Democratic Republic of Congo has observed a decline, as africanews reported on October 31, 2025.

• Transmission Dynamics and Risk Groups: Clade Ib mpox is believed to spread more readily from person to person, primarily through sexual or close physical contact. The WHO has specifically noted its detection among men who have sex with men (MSM), indicating community spread within these networks. While the overall risk for the general population remains low, it is assessed as moderate for MSM, particularly those with untreated HIV, according to vax-before-travel.

• Challenges in Detection and Surveillance: The appearance of unlinked cases in California and confirmed local transmission in European countries such as Spain, Italy, the Netherlands, and Portugal underscore significant challenges for public health officials. The absence of recent international travel history in some cases suggests the virus is circulating undetected, complicating rapid identification and containment efforts, as observed by The Guardian.

• Vaccination and Prevention Strategies: Vaccination remains a crucial tool in preventing the spread of mpox. The JYNNEOS vaccine, approved for use in the United States, is recommended for individuals at higher risk. Studies have demonstrated that a two-dose series is highly effective, with the CDC reporting 85.9% effectiveness against medically attended mpox disease. The UK Health Security Agency (UKHSA) anticipates some level of protection against clade Ib, though specific efficacy studies are ongoing.

• Clinical Presentation and Severity: Typical mpox symptoms include a characteristic rash or lesions, accompanied by fever, headache, muscle pain, and swollen lymph nodes. Historically, Clade I has been associated with more severe illness; however, recent data from the CDC suggest that Clade Ib infections might not be as clinically severe as previous Clade I outbreaks. Nevertheless, the variant has been linked to more intense symptoms and a higher fatality rate in vulnerable populations, including children and immunocompromised individuals, as noted by Symptoms of Mpox.

• International Response and Preparedness: The global health community, spearheaded by organizations like the WHO and ECDC, is prioritizing heightened surveillance, improved vaccine access, and stronger infection control measures. The US Department of Health and Human Services and the CDC have been actively preparing for potential clade I mpox cases since December 2023, establishing incident response structures and enhancing surveillance capabilities, as detailed by the CDC.

• Future Outlook and Research Needs: The quiet spread of clade Ib across continents and its adaptation to human-to-human transmission highlight the urgent need for global collaboration. Ongoing research aims to better understand the transmissibility and clinical severity of clade Ib compared to other variants, and to refine existing prevention and treatment strategies. Wastewater surveillance is also being explored as an effective tool for early detection of hotspots, according to The Journal of Infectious Diseases.