New deadly strain of mpox found in DRC could spread exponentially among humans

Concerns are growing about an outbreak of deadly mpox caused by a novel, sexually transmitted strain of the virus in the southeast of the Democratic Republic of Congo (DRC).

The strain triggered an outbreak in September, which has persisted and has now jumped to another province.

There are concerns that it might already have crossed borders and be spreading silently elsewhere, in the same way as the strain of mpox that caused the global epidemic in 2022—yet this time it appears the new strain is significantly more lethal.

To contain it, DRC and its neighbors need to acquire the former smallpox vaccines which have been shown to be effective against mpox. These are available, and DRC last week put regulatory approval in place enabling it to use donated vaccines.

In April, the US development agency USAID offered DRC 50,000 doses of a vaccine from the US stockpile that was developed for smallpox but works against mpox. Yet, that is barely enough for 25,000 people.

While the vaccine approvals mean DRC can now accept and use such donations, it cannot get anywhere like the number of doses it needs through agencies such as GAVI, the vaccine alliance, which help low-income countries buy vaccines.

First the vaccines, which cost US$200 for a course of two injections, according to Rosamund Lewis, emergency manager for the global mpox response at the WHO, need prequalification from the UN health agency for use against mpox, a long process not yet under way.

Emergency measures

A WHO Emergency Use Listing would also enable purchases, but to date the WHO has only issued these when a Public Health Emergency of International Concern has been declared for the outbreak.

One was issued in 2022 for the global mpox epidemic, but it was lifted in May 2023 when behavioural change and immunity from vaccination or exposure among the people most affected—men who have sex with men—had reduced case numbers.

One is not in place for mpox now, but as the virus spreads into crowded refugee camps, some form of emergency vaccine authorisation may be needed soon.

GAVI plans to set up a global stockpile of mpox vaccines, while the WHO’s Strategic Framework for mpox calls for a global rapid vaccine deployment mechanism by 2026.

This may not be possible, however, until use of the vaccines for mpox has either undergone a slow WHO approval process called prequalification, or benefits from some form of emergency use policy that also confers WHO backing, so lower-income countries can get assistance to access them.

The sexually transmitted strain is only a small part of the mpox epidemic now afflicting the entire country. But unlike mpox elsewhere in DRC, it circulates only in humans, meaning case numbers could explode.

This makes it worryingly similar to the first sexually transmitted mpox virus ever seen, which erupted into an epidemic in 2022 that has reached at least 96,000 people in 116 countries, most of them previously mpox-free. This strain continues to circulate at low levels worldwide.

Formerly known as monkeypox, symptoms of the disease include a rash that often resembles blisters alongside fever, aches and swollen glands.

Global threat

Normally people get mpox in Africa from local small mammals. Such cases are also increasing in DRC, as well as the sexually transmitted strain.

However, the West African strain of the virus that caused the global outbreak in 2022, and the new DRC strain, have both mutated so they can undergo sustained sexual transmission exclusively in humans.

That means they can pose a global threat.

On 24 June, after a long regulatory effort by Congolese experts, the World Health Organization and others, the DRC approved two mpox vaccines, the second African country after Nigeria to do so.

In theory, that means the spread of the new strain can be contained if, in addition to other health measures, enough people in the affected area are vaccinated.

“That’s how we eradicated smallpox,” once a far more widespread disease, says WHO’s Lewis.

However, that could be difficult in the DRC. The region with the new strain is wracked by armed violence, and people—including sex workers, who are heavily affected—frequently move cross-country and cross-border.

Cases have already been found in a crowded refugee camp in Goma, North Kivu, and there are fears of spread to neighbouring Uganda, Rwanda and Burundi.

Success will depend on how quickly the DRC can get vaccines and set up an immunisation drive, which must take in the whole country as far more people, especially children, are dying outside North and South Kivu of the normal type of mpox caught from animals.

Mpox is closely related to the smallpox virus, for which vaccination used to be widespread worldwide, including in Africa. The WHO-led vaccination drive eradicated the virus in the wild in 1980. Vaccination then ceased, leaving the ever-growing number of people born since then with no immunity to mpox.

The animal-borne Clade II virus found in Nigeria gave rise in 2016 to Clade IIb, which carried mutations caused by a human enzyme called APOBEC3, showing it had spread exclusively human-to-human since 2016. This happened both within and outside Africa.

The human-to-human spread meant cases rose exponentially and exploded in 2022 when the virus went global, with a death rate of  0.2 per cent. By comparison, the pre-vaccine death rate from COVID-19 averaged 1.7 per cent.

Separately, cases of Clade I, the type of mpox found in DRC, that were caught from animals started rising steeply in December 2022. Cases hit a record 15,000 in 2023 – most likely only the tip of the iceberg, say experts, given the DRC’s stretched healthcare services. This year the numbers are on track to almost double.

Death rates from this form of the disease are between 4.5 and 6.5 per cent, causing significant concern among the global health community.

Clade Ib

South Kivu province in eastern DRC had not had any mpox for decades. But last September, cases started spreading in the densely populated mining region of Kamituga, then to the rest of South Kivu. Nine cases have now been found in the city of Goma in North Kivu.

They were being caused by a virus, now named Clade Ib, identified in April 2024, which also has many APOBEC3 mutations, showing it has circulated exclusively in humans for several years. The largest groups infected in South Kivu have been adult men and women, especially sex workers, suggesting it is spreading heterosexually.

This means it too could stage a global outbreak, if eventually numbers rise exponentially as they did with the Nigerian strain. The death rate from this virus is so far unclear, but early estimates are much higher than the 2022 epidemic.

“An mpox outbreak anywhere is a threat everywhere,” said Jean Kaseya, head of the African Centres for Disease Control and Prevention, on 13 June. He called for “swift and urgent action” to improve Africa’s access to vaccines, diagnostics and anti-viral drugs.

Enabling WHO emergency use listing of the vaccine would seem the obvious way for such a rapid response to be achieved.