Faster spreading strain of mpox raises alarm in Congo

Editor’s note: This report has been updated to clarify a Congo province referred to by Sylvie Jonckheere.

Global health authorities are raising the alarm over a strain of mpox spreading through the Democratic Republic of Congo, with little known about the mutation other than it seems to spread more easily among humans. 

The mpox virus circulating in the North and South Kivu provinces of Congo is believed to be mutated from the lineage — clade I — that is endemic to Central Africa, distinct from the strain — descended from clade II — that impacted the U.S. and other Western countries in 2022 into 2023. 

According to the World Health Organization (WHO), the risk associated with mpox in Congo remains high, with the recently identified mpox strain estimated to have emerged around September 2023. 

While more is still being learned, the virus does appear to have some worrying features. 

“The main difference and what we see or can confirm with epidemiology is there is a sustained human-to-human transmission. It’s been going on now for months, and that’s really new as far as clade I goes,” said Sylvie Jonckheere, emerging infectious diseases adviser for Doctors Without Borders who is currently based in Goma, Congo. 

Clade I mpox is primarily spread from animals to humans as a zoonotic disease and is generally considered to have a higher mortality rate than clade II, causing death in up to 10 percent of infections. 

The WHO noted the clade I-descended strain has mutations that indicate “adaptation of the virus due to circulation among humans.” Unlike COVID-19, the mpox virus is not known for mutating rapidly. 

William Schaffner, spokesperson for the National Foundation for Infectious Diseases, said mpox is more akin to measles — for which one vaccine course is usually sufficient for an entire lifetime — in terms of how easily it mutates. 

“Certainly, a large splurge, a large outbreak of mpox such as we’re seeing now has not previously been recorded. There have been sporadic cases, of course, but nothing like this,” Schaffner said. “This is a very distinctive outbreak. It seems not to be a virus that mutates readily, but we’ve seen what I’m sure is a consequence of a mutation.” 

Unlike the form of mpox that spread through Western countries in 2022, the newer strain does not appear to have a strong link to sexual transmission or the social networks of men who have sex with men.  

According to Jonckheere, roughly 25 percent of cases are occurring among children. But the mpox virus circulating through eastern Congo does not appear to cause more severe illness. 

“It’s actually not more severe, the case fatality rate. So, the lethality of this disease for now in the eastern part of Congo is still rather low compared to what we saw in Équateur, historically or even recently,” Jonckheere said, referring to a province in Congo.

Outlets such as the BBC have reported that this mpox mutation could be the “most dangerous strain yet.” Jonckheere said it depends on “how you define ‘dangerous,’” noting that the current lack of comprehensive information on this mutated strain warrants “a pinch of salt, maybe a bit more than a pinch.” 

“It seems that it’s likelier to be transmitted from a human to a human. It will follow humans wherever they go,” she said. “In this part of Congo, there’s a lot of movement between countries also, so it could be spread further afield. But if you look at case fatalities or mortality or these kind of things, it’s not more dangerous. So, it really depends on how you define it.” 

One factor that isn’t helping the outbreak is an inability to sufficiently socially distance.  

“There’s no option for distancing in any way. People are living, you know, seven in a four meter square shelter, literally,” said Jonckheere, who noted a lack of clean water, as well as sexual or transactional contact, as other factors worsening the mpox spread in the region. 

As of now, this mpox strain appears confined in Congo, though the region it’s located in borders both Burundi and Rwanda, providing opportunity for infections outside of the country. 

Jonckheere noted its transmission is being assessed as a regional threat. 

Vaccination efforts are being made in Congo. Schaffner noted the smallpox vaccine deployed in the U.S. during the 2022 mpox outbreak should still be effective against the mutated strain. 

While mpox is no longer considered to be a public health emergency in the U.S., the WHO still considers there to be a “moderate” threat in countries with historical transmission and their neighbors. The general population in countries not affected prior to the outbreak that began in 2022 is considered low risk. 

A WHO report in June listed 646 laboratory-confirmed cases of all types of mpox reported from 26 countries in May, an increase over the prior month. 

As reporting on the virus has declined since 2022, the current estimate of cases is believed to be an underestimate. 

“It continues to be a concern. I mean, the epidemic curve has started to increase, peaked and has come way, way, way down, but it never disappeared,” Schaffner said. “There haven’t been, I think, any reports in the adjacent countries. And there are no reports so far of any exportations to Europe, Canada, the United States or elsewhere. But obviously, public health is very keenly following this because it has that potential.” 

Updated on July 9 at 10:49 a.m. EDT