London School of Hygiene & Tropical Medicine

A new strain of clade I mpox virus has been identified in human-to-human transmission within the Democratic Republic of the Congo (DRC) and is currently being monitored, according to the World Health Organization (WHO).

Mpox is an infectious disease with similar but less severe symptoms to smallpox, including fever, swelling of the lymph nodes, pain and skin lesions. 

Outbreaks of mpox typically occur through spillover from animals to humans. Cases are often found close to rainforests where species of rodents are known to carry the virus which causes the disease, referred to as mpxv. However, transmission can also occur between humans through lesion-to-skin contact, including contact during sexual activity. It is also possible it may be transmitted through respiratory droplets. 

The new clade I virus strain, identified as having novel mutations, is estimated to have emerged around mid-September 2023 and has continued to circulate from human-to-human. It is not known whether it is more transmissible or leads to more severe disease than other clade I mpox strains. 

Exploring factors which may be contributing to the outbreak, Michael Marks, Professor of Medicine at the London School of Hygiene & Tropical Medicine (LSHTM), said:

“Broadly, there are two major clades of mpxv – clade I and clade II. The 2022 global mpox outbreak was caused by clade II, the main clade circulating in West Africa. Clade I, the clade circulating in the DRC, is typically found in Central Africa.

"Clade I has historically been associated with a higher mortality rate but it remains unclear to what extent this is driven by differences in the virus itself, or by external factors such as the populations it affects and their access to healthcare.

"In some cases, mpox infection can clearly be very severe and lead to loss of life. What really remains uncertain is what proportion of milder cases are going undetected, as if only the most unwell cases are being identified, it will seem that all cases are severe. So while we’re currently seeing a lot of acute cases in the DRC, it could be because this is a much larger outbreak, resulting in more cases being identified, or it could be because this particular strain is more severe. 

“There is no dedicated vaccine against mpox approved worldwide but during the 2022 outbreak, the smallpox vaccine was used successfully to help protect vulnerable groups due to similarities between the viruses. Researchers are also looking into whether the use of antivirals may be effective treatments.

“Given transmission is predominantly via lesion to-skin-contact, the major risk is to populations locally and in neighbouring countries. We therefore need to address the major ongoing issue of vaccine inequity. While we widely rolled out vaccination in the 2022 outbreak to many groups, there continues to be huge populations at risk in the DRC and other African countries who can’t access the vaccines they need.

“Of course there is some risk of outward transmission but clearly the priority and focus needs to be on controlling the local epidemic. We really need to get a better handle on the true number of cases across all stages of severity to understand and tackle this outbreak effectively."

David Heymann, Professor of Infectious Disease Epidemiology at LSHTM, said:

“Management and detection of mpox outbreaks can often be challenging. 

“During the clade II outbreak that occurred outside of West Africa in 2022, many factors contributed to transmission across and within countries, including travel by those who did not know they were infected, either because their symptoms were mild or asymptomatic; avoidance of reporting due to social stigma; and poor access to treatment. 

“While infection with the clade II mpox virus typically results in mild symptoms, the clade I strain circulating within the DRC has an estimated mortality rate of 10%. This is an estimate based on reported cases, which are often the most severe. In comparison, the estimated mortality rate for Covid-19 in New York City was estimated as 1.4%.

“Attention must be paid to clade I outbreaks in DRC, where transmission patterns appear to be changing with human-to-human transmission. Human-to-human transmission was low when communities were vaccinated against smallpox but populations under 40 years of age, born since smallpox vaccination was discontinued in DRC and all other countries, are now at great risk. 

“Strategies for vaccination to prevent and contain outbreaks of clade I mpox must be understood by field study including the ring vaccination strategy used to contain smallpox outbreaks, and more general vaccination strategies to protect against emergence or human-to-human transmission outside outbreak sites. 

“There are robust surveillance systems in place for viruses worldwide to detect mpox in humans, and there is also a good understanding of the vaccination strategies needed to protect vulnerable groups from clade II infection, such as those living with HIV who are at higher risk of developing severe infection and should be prioritised. But little research has been done to prevent or contain outbreaks of clade I mpox in DRC, where infection, disease and mortality continue to increase each year. 

“While any emergence of a new strain is concerning, it is also an opportunity to gain better understanding of the virus and how it transmits, which in turn improves surveillance and future vaccine development. Much more effort must be made to understand the epidemiology and vaccination strategy for clade 1 mpox in DRC in order to protect human life there, and in other countries to which clade I mpox might be introduced.”