Dr Susan Hopkins, of the UK Health Security Agency (UKHSA), said updated figures for the number of confirmed cases will be released on Monday – with 20 cases confirmed so far.
She told BBC One’s Sunday Morning the rare virus was “absolutely” spreading via community transmission – the term used to describe when a person is infected without having been overseas or having been in recent contact with a confirmed case.
Cases of the smallpox-related disease had previously been seen only among people with links to central and West Africa, but now the UK, Spain, Portugal, Italy, the US, Sweden, Canada, and Israel have all reported infections.
The World Health Organization (WHO) has identified about 80 cases globally, and roughly 50 more suspected cases.
Health Secretary Sajid Javid announced on Friday the government would be stocking up on vaccines that are effective against monkeypox.
“The government has some stocks of the smallpox vaccine, which can be effective against monkeypox as the viruses are quite similar,” he said.
Dr Hopkins told the BBC “there is no direct vaccine for monkeypox” but added: “We are using a form of smallpox vaccine – a third-generation smallpox vaccine that’s safe in individuals who are contacts of cases.”
She explained the vaccine was only being given to “individuals who we believe are at high risk of developing symptoms” rather than the general population.
“For contacts, [this] reduces your risk of developing disease, so that’s how we’re focusing our vaccination efforts at this point,” she added.
Dr Hopkins said community transmission of the virus has been “largely centred in urban areas,” adding: “We are predominantly seeing it in individuals who self-identify as gay or bisexual, or other men who have sex with men.”
Monkeypox is a hard virus to transmit between humans, experts say. It needs very close contact, for example, skin to skin contact with an individual who is infected with a monkeypox rash.
We would recommend to anyone who’s having changes in sex partners regularly or having close contact with individuals that they don’t know to come forward if they develop a rash,” Dr Hopkins said.
It comes as US president Joe Biden said that recent cases of monkeypox which have been identified in Europe and the United States are something “to be concerned about”.
In his first public comments on the disease, Mr Biden added: “It is a concern in that if it were to spread it would be consequential.”