COVID-19 has been called a "once in a century" pandemic, but new research suggests that the kind of event thought to have sparked its outbreak - the spillover of a bat virus to people - is more common than previously known.
Some 400,000 people across Southeast Asia and southern China may be infected by Sars-related bat coronaviruses on average each year, though most infections go undetected and may not spread, according to research from a team of emerging infectious disease specialists.
"It seems like a huge number, but when you think about the number of people that live in that region - it's hundreds of millions of people with a very active wildlife trade, high exposure to wildlife and tens of millions of bats flying out every night ... and eventually you get infected," co-author Peter Daszak said.
"Then it's only a matter of time that one of those viruses is able to take off (into an outbreak)."
The findings, which have yet to be peer reviewed, were released on a preprint server this week.
The researchers acknowledged that the estimate was limited by the lack of solid data on the frequency of such infections, but said the study was the "first published attempt to identify the rate of spillover" for these novel bat coronaviruses.
The team included Linfa Wang of the Duke-NUS Medical School in Singapore, who, like Daszak, was involved in tracing the pathogen that caused the 2002 Sars outbreak to bats. Daszak, president of the US-based EcoHealth Alliance, was part of a World Health Organization mission to investigate the origins of Covid-19 in the central Chinese city of Wuhan, where the first cases were identified.
To come up with the estimate, the scientists created maps showing the overlap between human populations and the habitats of 23 bat species known to carry coronaviruses related to the ones that cause Covid-19 and severe acute respiratory syndrome.
They then projected the number of infections in these areas using existing research on bat-human contacts and the findings of small surveys that looked at the percentage of a population with antibodies to Sars-related viruses.
But they cautioned that the 400,000 average annual infections estimate was based on limited surveillance data on the actual prevalence of these viruses in people and had a huge confidence interval of 1 to 35 million annual infections. The data, which focused on bats, might also miss potential exposure from other kinds of wild animals.
Experts not involved in the study said while there were limitations, the estimate still offered insight.
Martin Hibberd, a professor of emerging infectious disease at the London School of Hygiene and Tropical Medicine, said the estimate took a "sophisticated analysis approach" to available data, but could be refined as better data and methods became available.
"It is very useful to have an initial estimate that the research community can explore and improve on," he said.
Meanwhile, when it came to shedding any light on the origins of Covid-19, Hibberd said the estimate "suggests that a natural infection of humans by bat viruses is an entirely feasible way for Sars coronaviruses to initiate an outbreak".
The research comes amid heated controversy surrounding the origins of Covid-19. Questions have been raised about whether the virus could have leaked from a laboratory studying related viruses in Wuhan - a scenario most scientists, including Daszak, feel is less likely than a natural spillover.
Meanwhile, Chinese officials have tried to put the focus on further research outside its borders, pointing to uncorroborated evidence of early spread in Europe and the Americas.
The latest data, which provide the "most detailed estimates to date" of the distribution of bat hosts of related coronaviruses, could also aid efforts to identify the geographic areas where the Covid-19 virus first spilled into people, according to the researchers.
"When you look at the maps, you can see that it's logical that a Sars-related coronavirus is most likely to spill over into people in those hottest of the hotspots, and that's south China, and it's the bordering countries like Laos, Myanmar and Vietnam," Daszak said.
"That's where you should look for the bat reservoirs of Sars-CoV-2 ... your best guess would be (the southern Chinese province of) Yunnan or neighboring countries."
In generating maps overlaying the range of host bats and human populations, the researchers found nearly 500 million people lived in the 4.5 million sq km of Laos, Cambodia, Thailand, Vietnam, Nepal, Bhutan, peninsular Malaysia, Myanmar, southern China, and the western islands of Indonesia where the host bats are found.
Daszak said people could become infected by bats through a number of activities, including hunting or consumption of wildlife and collecting or using bat guano for fertiliser without wearing protective gear.
But while there might be a high number of infections, most viral strains would not replicate well in people, and even if a well-adapted strain was able to spread, it may just die out after a short chain of transmission, he said.
Even people who had symptoms might assume they had flu or a cold.
And yet, estimating the scale and hotspots for these infections could also help prevent and detect future spillovers of potentially dangerous viruses, according to the researchers.
"Our approach provides proof of concept for systematic risk assessment of zoonotic spillover, and a strategy to identify key geographic areas that can be prioritized for targeted surveillance of wildlife, livestock, and humans," they wrote.
"If used to target future surveillance and disease control, (it) may help to reduce the risk of future Covid-like outbreaks."
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