Li took an unusual approach to investigate the virus mutation. She analysed the viral strains isolated from 11 randomly chosen Covid-19 patients from Hangzhou in the eastern province of Zhejiang, and then tested how efficiently they could infect and kill cells.

The deadliest mutations in the Zhejiang patients had also been found in most patients across Europe, while the milder strains were the predominant varieties found in parts of the United States, such as Washington state, according to their paper.

A separate study had found that New York strains had been imported from Europe. The death rate in New York was similar to that in many European countries, if not worse…

This finding could shed light on differences in regional mortality. The pandemic’s infection and death rates vary from one country to another, and many explanations have been proposed.