WHO Europe has forecast just over 27,000 monkeypox cases in 88 countries by August 2, up from 17,800 cases in nearly 70 countries at the latest count.
Cases of the virus, which has been present in parts of Africa for decades, first began to be reported outside countries where it is endemic in May. It generally causes moderate symptoms, including fever, fatigue and painful skin lesions.
Anne Rimoin, an epidemiology professor at the University of California, Los Angeles, told Reuters that health officials “have to get in front of this”.
“It’s clear the window of opportunity for doing so is closing,” she added.
Prof Rimoin is a member of the WHO’s expert committee on monkeypox that met last week to determine whether the outbreak constituted a global health emergency.
Despite a majority of committee members voting against the move, WHO Director-General declared an emergency anyway.
Health experts have urged authorities to ramp up vaccination, testing, contact tracing and quarantine measures for those infected.
Antoine Flahault, director of the Institute of Global Health at the University of Geneva and chair of the WHO Europe advisory group, said that transmission is “clearly unchecked”.
The group has modelled three scenarios for the coming months. These include “sustained transmission”, either between men who have sex with men; beyond these groups and possibly into more vulnerable populations, like children, or between humans and animals.
The latter scenario risks the establishment of a monkeypox reservoir in animals in new countries, as it has in parts of west and central Africa, said Prof Flahault.
Ongoing transmission could also lead to mutations that make the virus more efficient at spreading in humans, scientists said.
The most likely route of monkeypox transmission is close physical contact, touching clothing, bedding or towels used by someone with the monkeypox rash, or touching monkeypox skin blisters or scabs.
Sexual intercourse is thought to expose people to a higher risk of contracting the disease as, although it is not known to be sexually transmitted, the close physical contact involved means exposure is more likely.
A majority of the cases seen so far have been in gay and bisexual men and men who have sex with men.