Public health officials are to announce more UK monkeypox cases on Monday, as efforts ramp up to contain the first multinational outbreak of the virus that has led to cases in at least 14 countries.

The unusual outbreak of the rare disease has sparked a wave of contact tracing and testing, with the closest contacts of confirmed cases – such as partners and people in the same household – offered a vaccine and told to isolate at home for up to 21 days.

Under UK Health Security Agency (UKHSA) guidance, the same high-risk contacts are advised to avoid immunosuppressed people, pregnant women and children under 12, as they are more vulnerable to serious infections. Reports have said that one of the UK cases is a child who is in intensive care in London with the disease. The NHS and UKHSA said they would not discuss individual cases.

While some monkeypox patients have been admitted to hospital, many confirmed cases that do not need specialist care are told to isolate at home until local health protection teams are confident they are no longer infectious. “The vast majority of identified cases are isolating at home and do not require hospital admission,” said Dr Claire Dewsnap, president of the British Association for Sexual Health and HIV.

Monkeypox does not spread easily, and most people who become infected develop only mild symptoms, including fever, rash and blisters, which can clear up without treatment. But the virus can cause more serious illness in those with weakened immune systems, pregnant women and young children.

Dr Susan Hopkins, chief medical adviser to the UKHSA, said further cases identified over the weekend would be confirmed on Monday. Since the first was announced in the UK on 7 May – a person who arrived in London from Nigeria – the agency has confirmed 19 more cases, the majority among young men who have sex with men.

The outbreak is unusual because although the UK has had sporadic cases of monkeypox before – seven from 2018 to 2021 – all previous cases were linked to travel from Nigeria. Most of the latest cases in the UK and beyond have no known links to parts of west or central Africa where the virus is endemic.

Globally, more than 180 confirmed or suspected cases are being investigated in at least 14 countries. More than half are in Spain and Portugal. Genetic analysis of three monkeypox viruses from the outbreak have found it closely matches the virus that spread from Nigeria in 2018 and 2019. The same studies suggest that a Belgian man contracted the virus on a recent trip to Portugal.

The pattern of the outbreak suggests the virus is spreading primarily through sexual networks. The infection can be passed on through close contact with body fluids, infected ulcers, and contaminated material such as towels and bedding. Superspreader events may have boosted the outbreak since it arrived in Europe.

“I think it’s possible that some of these outbreaks will be driven by superspreading events,” said Dr Michael Head, a senior research fellow in global health at Southampton University. “For example, the Spanish health authorities are looking at a sauna as a site of single exposure that resulted in many secondary cases. The reportedly mild presentations of several cases may also be a factor, in that people will be more mobile and potentially likely to socialise. However, we do still need to understand more about the transmission dynamics.”

Hopkins told BBC One’s Sunday Morning show that community transmission in the UK was “largely centred in urban areas and we are predominantly seeing it in individuals who self-identify as gay or bisexual, or other men who have sex with men”. Asked why cases were mostly in that group, she said: “That’s because of the frequent close contacts they may have.

“We are detecting more cases on a daily basis and I’d like to thank all of those people who are coming forward for testing to sexual health clinics, to the GPs and emergency department. We are finding cases that have no identified contact with an individual from west Africa, which is what we’ve seen previously in this country.

“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.”

Dewsnap said sexual health clinics were already under massive pressures because of cuts and lack of support in recent years, while there were a “worrying number of unfilled genitourinary medicine specialist posts in sexual health clinics.”