Chinese authorities have admitted for the first time that limited human-to-human transmission of the deadly H7N9 flu virus is theoretically possible, adding to fears over the virus that has now claimed 17 lives in eastern China.

In a departure from earlier comments, scientists at Beijing’s Center for Disease Control and Prevention said evidence from H7N9 infections in families was “unclear” and they were still studying the cases to determine whether human-to-human transmission had occurred.

“For a virus that can pass from birds to humans, then after a person is infected, in theory it can also infect, in very rare cases, members of a population who are particularly susceptible to infection or who have certain characteristics,” said Feng Zijian, director of the public health emergency centre at the CDC, in remarks posted on the Ministry of Health’s website.