International researchers, including from DTU National Food Institute, warn that the ongoing mpox outbreak in the Democratic Republic of the Congo (DRC) has the potential to spread across borders more rapidly. The mpox virus has mutated, and the new variant, clade 1b, has become more infectious.
Genetic analyses of clade 1b, first detected in September 2023 in Kamituga, DRC, show that this variant has since undergone mutations making it more easily transmissible between humans. Scientists have identified three new subvariants, one of which has spread beyond Kamituga to other cities in the DRC, neighbouring countries, and even internationally to e.g. Sweden and Thailand. The new data may also suggest that clade 1b entails a high risk of miscarriage.
This new research has been published as an accelerated scientific publication in the esteemed journal Nature Medicine.
Originally, mpox was considered a zoonotic disease that primarily spread from animals to humans. However, in 2022, the world witnessed an outbreak primarily affecting men who have sex with men. The new clade 1b differs as both men and women are now contracting the virus. Additionally, an increasing number of infections are being reported among healthcare workers and children.
It’s somewhat like SARS-CoV-2 – the virus undergoes mutations as it spreads. For clade 1b, we see that one particular subvariant appears to have become better at transmitting between humans, and it has now been detected in several countries outside East Africa. In addition, the number of infected pregnant women who miscarry is high among those we have tested.”
Professor Frank Møller Aarestrup from DTU National Food Institute, who leads the GREAT-LIFE project
The GREAT-LIFE project is behind both the discovery of clade 1b and the development of a new PCR test, which enables the detection of clade 1b—undetectable by the original mpox tests.
Call for cross-border collaboration to contain mpox spread
This research indicates that the new variant is spreading rapidly, primarily through heterosexual contact in densely populated areas.
“At present, we are seeing uncontrolled transmission of clade 1b in eastern DRC and Burundi, but to a lesser extent in other parts of East Africa. While there is some international spread, we do not yet expect a large outbreak outside the epicentre in East Africa. However, this situation demands immediate attention. It is crucial to avoid close contact, particularly sexual contact, in high-risk areas,” says Frank Aarestrup.
The spread to neighboring countries underscores the need for expanded cross-border cooperation to track disease transmission, treat patients, and disseminate health education, particularly among sex workers.
“Action is needed locally, including increased vaccination efforts and public awareness campaigns on transmission routes. Additionally, global measures could include travel advisories against visiting high-risk areas and particularly against engaging in sexual contact in affected regions,” says Frank Aarestrup.
The GREAT-LIFE project coincided with mpox outbreak in DRC
DTU National Food Institute coordinates the GREAT-LIFE project, which aims to build capacity for detecting disease outbreaks in East Africa. The project focuses on local implementation of PCR testing for viral diseases using portable equipment. This capacity-building initiative was rapidly tested when, by coincidence, the project launched alongside the emergence of the new clade 1b variant in the DRC.
The mpox outbreak meant that local researchers and healthcare personnel immediately needed the expertise and tools provided by the project. Led by Professor Frank Aarestrup, DTU National Food Institute’s contribution is to equip local researchers with the ability to conduct rapid research and deliver relevant results. As part of its capacity-building efforts, the GREAT-LIFE project has:
- As the first, identified the new variant of the mpox virus: Ongoing mpox outbreak in Kamituga, South Kivu province, associated with monkeypox virus of a novel Clade I sub-lineage, Democratic Republic of the Congo, 2024 – PubMed
- Developed a PCR test to detect clade 1b: Real-time PCR assay to detect the novel Clade Ib monkeypox virus, September 2023 to May 2024 – PubMed
- Tracked and investigated its spread to Burundi: Monkeypox Clade Ib virus introduction into Burundi: first findings, July to mid-August 2024 – PubMed
- Examined the spread in and beyond Kamituga, DRC, as detailed in this article.
Key findings
As of 5 January 2025, more than 9,500 individuals have tested positive for mpox in the Democratic Republic of the Congo (DRC), with an estimated fatality rate of 3.4%. The rapid increase in cases in the South Kivu province of the DRC is particularly concerning.
In this regard, this research shows that:
- The mpox virus has become more transmissible, leading to faster spread.
- Transmission occurs primarily through heterosexual contact.
- The virus spreads through sex workers in densely populated areas.
- There is significant underreporting of cases.
- Mpox infection likely increases the risk of miscarriage in pregnant women.
The researchers have analysed samples from 670 patients infected with mpox. Their findings indicate that 52.4% of those infected were women, while 47.6% were men. The majority of infections were transmitted through sexual contact, but three cases were recorded among healthcare personnel. Seven patients died, and eight out of 14 pregnant women suffered miscarriages.
The research focused on South Kivu province in the DRC, where clade 1b transmission began in September 2023.
About the scientific article
The article has been published in Nature Medicine under a clause allowing rapid dissemination of the findings.
Original title: Epidemiological and genomic evolution of the ongoing outbreak of clade Ib mpox virus in the eastern Democratic Republic of the Congo.
It has been authored by 16 researchers from six different countries: DRC, Rwanda, Denmark, the United Kingdom, Spain, and the Netherlands.
The research has been funded by the EU and EDCTP under the Global Health EDCTP3 grant agreement No. 101103059 (Greatlife project); the EU Horizon 2020 grant VEO 874735; DURABLE (HERA-funded network); Wildlife Conservation Network (WCN); Conservation Action Research Network (CARN); and with support from the Greatlife Mpox Consortium.
Masirika, L. M., et al. (2025). Epidemiological and genomic evolution of the ongoing outbreak of clade Ib mpox virus in the eastern Democratic Republic of the Congo. Nature Medicine. doi.org/10.1038/s41591-025-03582-1.
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