When Xiaolu Tang at Peking University in Beijing and colleagues studied the viral genome taken from 103 cases, they found common mutations at two locations on the genome. The team identified two types of the virus based on differences in the genome at these two regions: 72 were considered to be the “L-type” and 29 were classed “S-type”.

A separate analysis by the team suggests that the L-type was derived from the older S-type. The first strain is likely to have emerged around the time the virus jumped from animals to humans. The second emerged soon after that, says the team. Both are involved in the current global outbreak. The fact that the L-type is more prevalent suggests that it is “more aggressive” than the S-type, the team say.

“There do appear to be two different strains,” says Ravinder Kanda at Oxford Brookes University in the UK. “[The L-type] might be more aggressive in transmitting itself, but we have no idea yet how these underlying genetic changes will relate to disease severity,” she says.