Now the virus has crossed from its epicentre in the Democratic Republic of Congo to other African nations and was detected this week for the first time in Sweden and Pakistan.
The disease, formerly known as monkeypox, was first detected in humans in the DRC in 1970.
There are two subtypes of the virus: clade 1 and clade 2.
The deadlier clade 1 has been endemic in the Congo Basin in central Africa for decades.
The less severe clade 2 has become endemic in parts of West Africa.
Mpox can spread human-to-human through sexual or close physical contact. Symptoms include fever, muscular aches and large boil-like skin lesions.
The virus gained international prominence in May 2022, when a less deadly strain called clade 2b spread around the world, mostly affecting gay and bisexual men.
Between January 2022 and June 2024, 208 deaths and more than 99,000 mpox cases were recorded across 116 countries, according to the WHO.
The latest surge has been of the deadlier clade 1 — and its new mutated variant.
The new strain, called clade 1b, was first detected among sex workers in the DRC in September 2023.
Sweden this week reported the first case of the variant outside Africa, and the EU’s health body urged countries to increase preparedness. Pakistan reported Asia’s first case.
“It is not surprising … that travel between continents has brought this case to Europe,” said Brian Ferguson, an Associate Professor of Immunology, University of Cambridge.
He added that cases were likely to spike in Europe and elsewhere as “there are currently no mechanisms in place to stop imported cases of mpox”.
Containing the epidemic will require “rapid international cooperation”, said Francois Balloux of the University College of London Genetics Institute — adding there was “no evidence of transmission in Europe at this stage”.
04:17
WHO declares mpox a global health emergency as new form of virus emerges
WHO declares mpox a global health emergency as new form of virus emerges
Clade 1 mpox is “known for causing more severe disease in young children, pregnant women, and immunocompromised people”, said Jonas Albarnaz, who specialises in pox viruses at the Pirbright Institute in Britain.
Clade 1b is driven by sexual transmission and mostly infects young adults, Albarnaz said.
It has also been recorded spreading through non-sexual contact between people, including children playing together at school.
Clade 1b causes death in around 3.6 per cent of cases, though infants and children are more at risk, according to the WHO.
More mpox cases were reported in the first half of this year than in all of 2023, according to WHO figures.
Most recent cases have been in the DRC, where 548 people have died so far this year, the government said.
Over the last month, previously unaffected countries such as Burundi, Kenya, Rwanda and Uganda have reported outbreaks, according to the Africa CDC. None have reported deaths, the WHO said.
Nigeria has flagged 39 cases of a milder strain of mpox this year, health officials said on Friday.
Sweden and Pakistan this week reported the first mpox cases outside Africa, with the WHO warning more cases of the new strain were likely in Europe.
During mpox’s global spread in 2022, vaccines were deployed in Europe and North America, which helped control the outbreak.
But vaccines have not been made widely available in the African countries most affected by mpox.
The US Department of Health said on Wednesday it would donate 50,000 doses of an mpox vaccine to DRC.
On Tuesday, Africa CDC head Jean Kaseya announced an agreement with the European Union and Danish drug maker Bavarian Nordic to distribute 200,000 doses across the continent.
Although that would not be enough, Africa could secure another 10 million vaccines, Kaseya told a media briefing.
Bavarian Nordic said on Friday it was seeking European approval to use its mpox vaccine in children aged 12 to 17.