Experts have warned that the coronavirus spreading around the world could change and become more virulent after they observed mutations in a cluster of cases in a family. Researchers in China sequenced the genome of several members of the same family all infected with the coronavirus, known as 2019-nCoV, and found that “viral evolution” had taken place during person-to-person transmission of the disease. Outlining their findings in the journal National Science Review the researchers said that close monitoring of the “virus’s mutation, evolution and adaptation” was necessary to ensure that the disease does not spread out of control. They warned that previous studies of viruses show that mutations are often associated with increasing spread of disease, “increases in virulence, the evasion of the host immune system and the evolution of resistance to antiviral treatments”. In a separate paper in the Journal of Virology researchers who have spent a decade studying severe acute respiratory syndrome (Sars) – the coronavirus genetically closest to this new disease – also warned that a single mutation could lead to a more serious outbreak.

They wrote: “2019-nCoV evolution in patients should be closely monitored for the emergence of novel mutations.” The warning comes as the latest figures show that the coronavirus – also known as 2019-nCoV – has infected 20,704 people and killed 427. The virus has also killed a second person outside mainland China – a 38-year-old man in Hong Kong – and has spread across the world including to the UK, Germany and India. The UK Foreign Office has urged UK residents in China to leave the country.

Dr Nathalie MacDermott, National Institutes of Health Research academic clinical lecturer at King’s College, London, said viruses mutate all the time – often mutations have little effect but they can make the virus stronger or more virulent. She said a single mutation of the Ebola virus during the 2014-16 epidemic in West Africa, where around 11,000 people died, meant the disease became more contagious. “During the Ebola outbreak there was one significant mutation… when the virus crossed into Liberia and Sierra Leone from Guinea there was a difference in the virus and it became more infectious to humans and less infectious to animals,” she said.

But while researchers need to be on the alert there is currently no evidence to suggest that significant mutations are taking place in the coronavirus. Dr Sylvie Briand, director of epidemic and pandemic diseases at the World Health Organization, told a press conference that the coronavirus currently appeared to be “stable”. Public Health England announced earlier today that the sequencing of the virus among the two patients infected in the UK show “that so far the virus has not evolved to better infect humans since the sequence was first published by China”. Experts say it is too early to say what effect any mutations will have in humans.

Jonathan Ball, professor of molecular virology at the University of Nottingham, said: “Until we can actually show the biological impact and the nature of those genetic changes, all you can say is the virus is evolving and mutating. So we know it mutates but we don’t know if it means anything.” Dr MacDermott added: “We won’t necessarily know the impact of these mutations until we have seen how the epidemic unfolds and if mortality rates change.” Data released by the Chinese authorities today show that the death rate from the disease across mainland China is 2.1 per cent but in Hubei province, the epicentre of the outbreak, the rate is 3.1 per cent.