In parts of rural China where bat guano is collected from caves and used as fertiliser, people had been found to be carrying antibodies to those viruses, indicating previous infection. In other words, a spillover event could have happened without the involvement of any lab.
Still, it remains possible that the virus escaped from a lab. Some lab leak theorists consider it suspicious that the initial outbreak of Covid-19 happened in Wuhan, home to several high-security labs that study coronaviruses.
But Robertson says the fact that Wuhan is a city is explanation enough. Unlike the rural areas where the virus might have infected people previously, Wuhan has the population density – 11 million people – to sustain an outbreak.
Of all the known bat coronaviruses, the most similar to Sars-CoV-2, sharing 96% of its genome, is RaTG13, a virus that researchers at the WIV were studying prior to the pandemic.
But since RaTG13 was identified, others have been isolated from bats in China and south-east Asia that are also highly similar to Sars-CoV-2. A cluster of them, from the Chinese province of Yunnan, was described last week in the journal Cell. “The connection between RaTG13, the Wuhan Institute of Virology and Sars-CoV-2 isn’t required any more,” says Robertson.