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Results of MOSAIC, a European observational study led by researchers at PSI and ISARIC, provides evidence on how the clade IIb mpox virus affects patients and the risks of onward transmission.

Clade I and II mpox virus is endemic in Central and West Africa, causing local seasonal outbreaks.  

In May 2022, a new variant of the mpox virus caused the first global outbreak, prompting the World Health Organization (WHO) to declare a Public Health Emergency of International Concern, and causing nearly 90,000 cases worldwide in one year. This variant, known as clade IIb, differs from the mpox typically seen in Africa, particularly in how it spreads and the symptoms it causes.  

With limited available clinical evidence about the disease’s symptoms and best treatments, researchers at PSI and the International Severe Acute Respiratory and Emerging Infection Consortium (ISARIC) embarked with partners across Europe on a programme of research to generate robust and clinically significant new data.  

Results from one of these studies on a large sample of participants, published in Clinical Infectious Diseases, provide insights into how clade IIb mpox affected people during the recent outbreak, the risks of onwards transmission and health outcomes for patients. 

 The observational study – known as MOSAIC and funded by the UK Foreign, Commonwealth and Development OfficeWellcomethe Bill & Melinda Gates Foundation and by the European Commission with the MPX-Response in the European Union – was conducted to better understand the clinical features of mpox and to measure how long it took for lesions to heal and for the virus to clear.   

Data were collected through hospital visits, phone interviews and self-completed questionnaires over six months of follow-up.  

The research was initiated early into the outbreak and included a cohort of 575 participants with confirmed mpox virus infection, recruited between May 2022 and July 2023 in six European countries (Belgium, France, Italy, Spain, Switzerland and the United Kingdom). 

Study participants were mostly men aged 30-45 years; nearly half were living with HIV. The majority could be cared for at home without specific treatments and without requiring hospitalisation.  

Only 10% of participants received putative antiviral treatment specifically for mpox – shortages in drug supplies meant only few selected patients could be offered those medications. The study design did not allow researchers to fully assess the effectiveness of the antiviral treatments, as it was not designed to compare treated and untreated groups.   

Researchers found that lesions were mostly affecting the skin around the genitals and the anus. Additionally, they discovered that it took time for lesions to heal and to shed the virus. Importantly, 5% of the participants still had viable virus in skin lesions, which means they could still transmit the infection 7-8 weeks after testing positive. They also found that there were no deaths, and no participants experienced a recurrence of infection within the six-month follow-up period. And finally, people living with HIV/AIDS had well-controlled infections and their presentation and outcomes did not differ from participants without HIV infection.   

Professor Piero Olliaro, Chief Investigator of MOSAIC and PSI researcher, said: ‘The evidence generated by this study will help clinicians manage cases of mpox and ensure public health experts can implement the right measures to address mpox outbreaks in the future. Our research findings underscore the need for continued research to improve responses to ongoing outbreaks.’  

Importantly, a new Public Health Emergency of International Concern has been underway since August 2024, caused by yet another variant of mpox – clade Ib – affecting primarily Central and East Africa.  

PSI and ISARIC are supporting groups working in Africa and elsewhere to build and sustain clinical research on mpox. This work includes the clinical characterisation protocol (CCP), a standardised Mpox Case Report Form (CRF) template, and analytical tools that were recently used in the First Observational Study on Clade Ib Mpox Transmission and Disease Severity in South Kivu. Find out more about the ISARIC mpox response on their website.

Read the full paper in Clinical Infectious Diseases: https://academic.oup.com/cid/advance-article/doi/10.1093/cid/ciae657/7942253