LONDON (REUTERS) - British authorities are recommending gay and bisexual men at higher risk of exposure to monkeypox should be offered a vaccine, as the outbreak of the viral disease gathers pace mostly in Europe.

Although anyone can contract monkeypox, data from the outbreak suggests the majority of transmission is occurring within the sexual networks of gay, bisexual and other men who have sex with men, according to UK Health Security Agency (UKHSA).

A person's eligibility would depend on a plethora of factors but would be similar to the criteria used to assess those eligible for pre-exposure prophylaxis (PrEP) - medicine taken by people at risk for HIV to prevent getting HIV from sex or injection drug use, UKHSA said on Tuesday (June 21).

Doctors may advise vaccination for someone who, for example, has several partners, participates in group sex or attends "sex on premises" venues, the agency added, noting that people are advised not to come forward for the vaccine until contacted.

Initially, UKHSA had recommended only close contacts of cases, including healthcare workers, be offered the vaccine, Imvanex.

Bavarian Nordic's vaccine is approved for smallpox in Europe, but has been used off-label in response to monkeypox cases.

More than 2,500 cases of monkeypox have been reported from over 35 countries outside of Africa, where the virus is endemic.

As of June 20, there were 793 laboratory confirmed cases in the United Kingdom.

The outbreak has triggered concern since the virus is rarely seen outside of Africa - and the majority of cases are not related to travel to the continent.

Scientists are scrambling to understand what is driving the current outbreak, its origins and whether anything about the virus has changed.

The monkeypox virus is understood to spread through close contact with an infected person, who may shed the virus via its hallmark skin lesions or large respiratory droplets.

Monkeypox is not currently defined as a sexually transmitted infection, which is caused by a pathogen that passes from one person to the next specifically in semen, vaginal secretions or other bodily fluids. But monkeypox can be passed on via intimate contact during sex.