Health authorities worldwide are on alert after cases of monkeypox were recorded in at least a dozen countries, the largest outbreak of the virus outside sub-Saharan Africa since the first human infection was recorded more than 50 years ago.

There were nearly 100 confirmed cases globally, as of May 21, according to the World Health Organization. On Monday, the UK Health Security Agency said it had detected 36 additional cases of monkeypox, bringing the total number of cases confirmed in England since May 7 to 56, the highest total recorded by any country.

Some public health agencies, including in the UK and Belgium, have told those infected and their close contacts to quarantine for three weeks. Governments are also stocking up on smallpox vaccine doses, which also protect against monkeypox, as part of plans to inoculate close contacts.

No deaths have been recorded in the current outbreak.

What is monkeypox and how dangerous is it?

Monkeypox is a virus that typically causes fever and distinctive skin lesions, similar to chickenpox. Most cases are mild and the illness usually clears without treatment within two to four weeks.

The most common means of transmission is from skin-to-skin contact with an infected person or an exchange of bodily fluids. The virus can also spread via respiratory droplets. A large proportion of cases identified in the latest outbreak have been among gay and bisexual men.

“It manifests mainly on the skin’s surface,” said David Heymann, professor of infectious disease epidemiology at London School of Hygiene and Tropical Medicine. “It’s not at all like Sars-Cov-2,” he said referring to the virus that causes Covid-19. “It rarely transmits via the respiratory route.”

The virus — discovered in laboratory monkeys in 1958 and in a human 12 years later — typically emerges from close contact with animals, particularly rodents, in sub-Saharan Africa.

While some of the early cases in the current outbreak were linked with travel to west Africa, health authorities in western nations have said the virus is now spreading in their communities.

There are two types of virus: the west African strain and the alternative Congo-Basin version. All cases so far from the latest outbreak have been identified as the west African strain.

Heymann said it was “fortunate” it was the west African version because it was much less deadly. The west African strain has a case fatality rate of 3.6 per cent, compared with 10.6 per cent for the Congo-Basin version.

The “majority” of deaths from the west African strain, which occurred in previous outbreaks, have been in severely immunocompromised individuals, often people with HIV, according to Geoffrey Smith, professor of pathology at Cambridge university. However, young children and pregnant women are also at increased risk.

What is driving the latest outbreak?

A large-scale monkeypox outbreak was possible across the west because of a drop-off in immunity to the virus, according to several studies.

The smallpox vaccine, which stopped being administered in the west in the decade leading up to the virus’s eradication in 1980, was up to 85 per cent effective in preventing a monkeypox infection, according to the WHO.

As a result, most over-50s have some immunity to monkeypox whereas younger people lack protection against infection.

“This was waiting to happen,” said Eskild Petersen, an emeritus professor of infectious diseases at Aarhus University in Denmark. “There’s a growing number of people — basically everyone below 45 years old — who have absolutely no immunity to the pox viruses, and that could be an explanation for an increasing number of cases.”

However, some experts have argued that the sustained human-to-human transmission could be the result of a mutation in the virus allowing it to spread more efficiently through either sexual or airborne routes.

Marc Van Ranst, a virology professor at University of Leuven in Belgium, said his laboratory had sequenced the virus from the latest outbreak and found a “handful of mutations”, although the effect they would have on viral transmission was unclear.

The longest chain of transmission has increased in recent years from six to nine person-to-person infections, according to the WHO. Van Ranst said this suggested that the virus was already “becoming more comfortable in human hosts”.

But monkeypox is less likely to mutate rapidly because it is a DNA virus rather than an RNA virus, such as Covid-19 or influenza. “Poxviruses are pretty stable viruses and they don’t change quickly,” said Smith.

What treatments are available?

Monkeypox usually clears on its own without specific treatment, except for the most complex cases.

The WHO, in its advice, has stressed the importance of letting rashes dry and protecting them once they appear. In the rare cases where treatment is needed, assistance is aimed at lessening symptoms such as fatigue, pain or fever.

The smallpox vaccine can also be used against monkeypox. One strategy outlined by UK and European health authorities focuses on ring vaccination, whereby close contacts of those who present with symptoms are immunised.

The WHO on Monday said products specifically designed to treat monkeypox were not widely available.

A smallpox vaccine made by Bavarian Nordic has been approved for use in the US against monkeypox. Paul Chaplin, chief executive of the Danish drugmaker, said the company had a stockpile of about 1.4mn doses.

“We have very limited stocks, that’s why we are talking to multiple health agencies, we’re doing our best to try and do what we can with the stocks we have available,” he explained. “We are producing more . . . but vaccine production is not something you turn on and off easily, it takes a while.”

How worried should we be?

The WHO and national health authorities have warned people to expect more cases of the virus, but most experts believe that health officials will be able to bring the outbreak under control.

The European Centre for Disease Prevention and Control said on Monday that the risk of transmission for people with multiple sexual partners was “high”, while the overall risk to the general population was low.

“I think this is something that will fizzle out: it doesn’t spread easily between people, there’s a vaccine to prevent it, there are drugs that treat it and it’s easily observed when somebody’s got it,” Smith explained.

“All those things make containment a lot easier than something like Covid where you don’t know if somebody is infected unless they get symptoms.”

In a worst-case scenario, Heymann said western countries could face an “insidious” epidemic that takes time to burn out.

“It would be false security to say we can easily snuff this out,” said Heymann. “We have to be very careful and we have to make sure people know they have the power themselves to prevent infection.”

But he played down comparisons with coronavirus. “It certainly won’t become a pandemic . . . but it could remain in human populations for a time to come.”

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