A novel bird flu virus that has killed one mainlander and left another critically ill has the potential to spread efficiently among humans, doctors warn. They say the fact that the H10N8 bug jumped to humans for the first time in December is an important warning of a possible pandemic. "The pandemic potential of this novel virus should not be underestimated," the research team headed by virologist Shu Yuelong from the Chinese Centre for Disease Control and Prevention in Beijing says in a paper in The Lancet medical journal, published yesterday. "[The latest case] reveals that the H10N8 virus has continued to circulate and may cause more human infections in the future." An expert in infectious diseases in Hong Kong, meanwhile, said tests for the H10 virus should be stepped up among flu patients and poultry. A 55-year-old woman critically ill in hospital in Nanchang, Jiangxi, was confirmed to have the virus on January 26. This followed the death of a 73-year-old woman there on December 6. The fifth novel influenza strain to emerge in 17 years, the virus has a worrying genetic profile and should be closely monitored, the researchers wrote. It appears to be able to infect tissue deep in the lung, they said. The warning stems from analysis of a virus sample taken from the woman who died. Her death from H10N8 was announced on December 18. The virus strain had previously been found only twice on the mainland - in a sample from a lake in Hunan in 2007, and in live poultry in Guangdong in 2012. The Lancet paper said the strain infecting the two humans was different from those two samples. University of Hong Kong microbiologist Yuen Kwok-yuen said it was important to monitor the activity of H10N8 in both poultry and humans as it was difficult to predict how an influenza virus would evolve.