Tens of thousands of people in Southeast Asia could be infected with coronaviruses related to SARS-CoV-2 each year, a study published yesterday (August 9) in Nature Communications estimates. The research, which first appeared as a preprint last September, analyzed the geographic ranges of 26 bat species and found their habitat overlapped regions where half a billion people live, representing an area larger than 5 million square kilometers, reports Reuters. Analyzing that data along with estimates of the number of people who exhibited detectable coronavirus antibodies predicted that approximately 66,000 potential infections occur each year.

Stephanie Seifert, a virus ecologist at Washington State University in Pullman who was not involved in the research, tells Nature that the work “highlights how often these viruses have the opportunity to spillover.”

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The study coauthors considered the geographic ranges of bats known to host SARS-related viruses—primarily horseshoe bats (family Rhinolophidae) and Old World leaf-nosed bats (family Hipposideridae). They found hot spots of potential spillover events in southern China, parts of Myanmar, and the Indonesian island of Java—where both bat and human populations are particularly dense, reports Nature.

Most of these SARS-related viruses don’t easily spread among humans or cause illness. But study coauthor Peter Daszak tells Nature that with enough infections “raining down on people, you will eventually get a pandemic.”

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However, Alice Hughes, a conservation biologist at the University of Hong Kong who was not involved in the work, tells Nature that this analysis relies on outdated and low-resolution geographical range data. “What they are trying to do is very valuable and needs to be done, but it has to be done with more finesse,” she says.

The study authors argue their research can focus spillover monitoring to high-risk regions in order to identify outbreaks sooner. Renata Muylaert, a disease ecologist at Massey University in New Zealand who was not involved in the research, agrees, telling Nature, “The article has considerable significance for surveillance.”

This analysis looked only at bat-to-human spillover events and did not consider infections that first transmit from bats to an intermediate animal and later to humans. Daszak tells Nature that there were limited data on that type of event, but that including them would have “massively increased the estimated risk of spillovers.”