While the World Health Organization unveiled a plan Monday to stop the transmission of mpox, scientists studying the virus said it is changing quickly, complicating efforts to combat its spread.

The six-month, $135 million plan calls for boosting surveillance, prevention and response. Strategic vaccination efforts will focus on individuals at the highest risk, “including close contacts of recent cases and healthcare workers, to interrupt transmission chains,” the plan states.

The World Health Organization is “significantly scaling up staff” in affected countries, it said.

But the WHO notes there are challenges in implementing the plan.

“Limited surveillance and diagnostic capacities, particularly in Africa, complicate understanding the outbreak’s true extent and hinder effective response measures,” according to the plan.

Although mpox has been endemic in parts of Africa for decades, a global outbreak was reported in 2022. It was brought under control by May of last year, according to WHO, but an offshoot of the virus emerged in September in Democratic Republic of Congo.

That variant has been spreading rapidly, with cases reported in four neighboring countries. It was also detected in Sweden and Thailand, among people with a travel history to Africa. The World Health Organization declared a public health emergency in mid-August.

More than 18,000 cases, with 615 deaths, have been reported in Congo alone. Eastern Congo and neighboring countries are listed in the plan as “high risk” areas, especially among vulnerable populations such as sex workers and people displaced by conflicts.

Scientists studying the disease in Africa, Europe and the United States told Reuters news agency that the new strain of the virus, known as clade 1b, is mutating faster than expected. Early cases were detected among sex workers and their clients in Congo, but the affected groups began to shift. Now the virus is found in households, and it is especially severe in children, immunocompromised individuals, and pregnant women, according to the WHO.

Dr. Dimie Ogoina, an infectious diseases expert at Niger Delta University Hospital in Nigeria, who chairs the WHO’s mpox emergency committee, told Reuters he worries that “in Africa, we are working blindly.”

“We don’t understand our outbreak very well, and if we don’t understand our outbreak very well, we will have difficulty addressing the problem in terms of transmission dynamics, the severity of the disease, risk factors of the disease,” he said. “And I worry about the fact that the virus seems to be mutating and producing new strains.”

Mpox belongs to the same family of viruses as smallpox but typically causes milder symptoms such as fever, chills and body aches. It mostly spreads through close skin-to-skin contact, including sexual intercourse. People with more serious cases can develop lesions on the face, hands, chest and genitals.

Material from The Associated Press and Reuters was used in this report.