WHEN humans and other species intermingle and viruses move between them, experts call that “spillover.” As humans move and seek new living spaces where wild animals live, and climate change shifts the boundaries of those habitats, scientists predict we will see more of these spillovers.

Coronaviruses, which are common in bats, are no exception. But most often, some intermediate animal is thought to bridge the transfer of the virus from bat to human. For example, the Middle East respiratory syndrome, or MERS, coronavirus likely moved from bats to camels, and then from camels to people.

Investigators who have worked on the controversial subject of how SARS-CoV-2 — the coronavirus that causes COVID-19 — made the jump from bats to humans have taken on the broader question of how often such leaps happen, especially directly between bats and people, and their estimate is striking.

According to a preprint study posted online Sept. 14, which hasn’t been peer-reviewed yet, as many as 400,000 people each year in South and Southeast Asia might pick up SARS-related coronaviruses directly from bats. The study focused on South Asia and Southeast Asia because of the high human-bat overlap there.

Most instances of these “undetected spillovers,” as the study authors call them, don’t ping public health radar because they simply fizzle out. The infections remain unrecorded, causing mild or no symptoms at all. The human immune system simply quashes them most of the time, leaving behind antibodies to the virus as evidence of the victory.

The researchers, led by Peter Daszak, a British zoologist and president of EcoHealth Alliance, used several sources of data to arrive at their estimate. One was geographic information about where bats and humans overlap in their habitats. Another source was human blood samples with telltale antibody signs of battling a coronavirus and information about how long those antibodies persisted. The investigators also collected information on how often bats and humans encounter each other. When they entered all of this information into calculations of the risk that humans might contract a virus from a bat, they arrived at their estimate of 400,000 such encounters each year.(SD-Agencies)