As many as 400,000 people are infected by coronaviruses carried by bats every year, according to an analysis mapping the risk of exposure across southern China and southeast Asia.
The paper, published as a preprint this week, is the first to estimate how many people living in the region unknowingly contract a bat coronavirus similar to Sars-Cov-2 every year.
Its high profile authors include Prof Linfa Wang, who played a leading role in efforts to trace the origins of Sars, and Dr Peter Daszak, a member of the World Health Organization team investigating the origins of Sars-Cov-2.
The paper’s findings, experts say, show the “clear and present danger” posed by bat viruses and the risk they could spark another pandemic.
In particular, southern China, north eastern Myanmar, Laos and northern Vietnam are considered the most high-risk “hotspots” for new pathogens to spill over to humans.
“This is a very interesting and useful study,” said Dr Eddie Holmes, an evolutionary biologist and virologist at the University of Sydney, who was not involved in the research.
“It neatly shows the clear and present danger posed by the spillover of bat viruses to humans. These exposures are continual and frequent, and clearly occur far more often than any exposure in a lab,” he told The Telegraph.
In the study, researchers from Duke-NUS Medical School in Singapore and the EcoHealth Alliance in the United States built on previous efforts to track how many people in rural China have antibodies to bat coronaviruses - in some places, this figure is as high as three per cent.
The team then combined ecological and epidemiological data with modelling of the spread of bats to map the risk of exposure to bat coronaviruses across China and south east Asia and estimate the rate of unreported spillover events.
They found that an average of 400,000 people are infected with a Sars-like coronavirus every year, though the median figure - which reflects the midpoint of a range of values - was more like 50,000, reflecting some uncertainty.
Most of these people probably do not realise they have been infected with a new bat virus and are likely to dismiss an illness as a bad cold or flu, researchers said.
“While the actual numbers are hard to estimate with accuracy, what this study shows is that humans are continually exposed to bat coronaviruses,” said Dr Holmes. “More outbreaks would not be a surprise.”
Prof Stuart Neil, a virologist at King's College London who also was not involved in the research, added that the paper “makes a powerful point” and suggests there is a “strong likelihood” that zoonotic infections are frequently occurring.
“The vast majority of these [cases] will go nowhere – the virus will not be well adapted to grow or spread in humans and that will be it,” he told The Telegraph. “But it only takes one of those 50,000 or so to get enough of a toehold for sustained human-to-human transmission - and you have a potential pandemic.”
Experts involved in the research said the hotspot maps could be used to help target surveillance in high risk areas to catch new outbreaks before they spread, and to inform efforts to work with communities to reduce high-risk behaviours - for instance visiting bat caves or hunting and eating wildlife.
But it could also help the hunt for the origins of the Covid-19 pandemic, by shining a spotlight on the places where Sars-Cov-2 is most likely to have jumped from bats to humans.
The paper comes amid continued debates about how the coronavirus first emerged, with arguments over whether the pathogen jumped from bats to humans via a natural spillover event or a lab-related accident. The issue has been at the heart of mounting geopolitical tensions.
A report by US intelligence services, done at the request of President Joe Biden, proved inconclusive, while experts involved in the original WHO inquiry have warned time is running out to conduct critical research that could offer answers.
Protect yourself and your family by learning more about Global Health Security