Monkeypox can spread before symptoms appear, British researchers said on Wednesday, providing the first evidence indicating the virus can be transmitted this way.
It was previously thought that monkeypox was almost entirely spread by people who were already sick, although pre-symptomatic transmission had not been ruled out and some routine screening had picked up cases without symptoms.
Monkeypox, a relatively mild viral illness that is endemic in several countries in western and central Africa, exploded around the world earlier this year, with outbreaks in dozens of countries where it had never previously appeared.
There have been almost 78,000 confirmed cases and 36 deaths since, according to the US Centers for Disease Control and Prevention. Although cases have peaked in many countries, this week the World Health Organization said the outbreak still represented a global health emergency.
The virus is known to spread through close contact and causes symptoms including fever, body aches and often painful pus-filled skin lesions.
To find out more about how monkeypox was transmitting in Britain, a team from the UK Health Security Agency used routine surveillance and contact tracing data for 2,746 people in the country who tested positive between May and August.
Their average age was 38, and 95% of the patients reported being gay, bisexual or men who have sex with men.
Researchers analysed the "serial interval" - the time from when symptoms began in the first case to when they began in a connected case - as well as the incubation period, the typical time from exposure to the virus to the onset of symptoms.
Using two statistical models, they found that the median serial interval was shorter than the median incubation period. This indicated that "considerable" transmission happened before the appearance or detection of symptoms has taken place, the researchers wrote in the paper published in the British Medical Journal.
Four days was the longest period before symptoms began that transmission was detected, and the team said up to 53% of transmission may have taken place before symptoms began.
The study raises questions over tackling monkeypox worldwide, including whether asking people to isolate when symptoms appear is enough to stop the virus spreading.
Many richer countries have vaccinated high-risk populations to curb the outbreak, but shots are limited and there are none available in Africa.
Independent experts said the findings could have important implications for global infection control, if supported by other studies.
"These are urgent questions that need answers," said Boghuma Kabisen Titanji, assistant professor of medicine at Emory University, Atlanta.
Other scientists said the work was robust but needed clinical data before it could be labeled definitive or applied globally.
"It's an important part of the transmission jigsaw," said Jake Dunning, senior fellow at the Pandemic Sciences Institute at Oxford University, "but personally I want to see it joined up with other pieces."