Chinese authorities have begun to speculate about the “rare” possibility of human-to-human transmission of the H7N9 bird flu strain, adding to fears over the deadly virus that has claimed 17 lives and infected 82 people.

Beijing’s Center for Disease Control and Prevention backpedaled from earlier comments that evidence of infections in families was “unclear”, reports the Financial Times.

“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 center at the CDC.

“Further investigations are still under way to figure out whether the family cluster involved human-to-human transmission,” Feng said, citing the “Li family” at the center of the investigation after a father and his son both fell ill and died.

If proven, the development would quickly raise concerns that the new strain could lead to a pandemic.

The CDC, however, downplayed fears that more humans would be susceptible to the disease. “Even if a small number of family cluster infections appear, it would not mean the virus had already achieved the ability of effective human-to-human transmission,” he said, adding that no firm evidence of human-to-human transmission had yet been found.

An international team of experts, including some from the World Health Organization, have been deployed to investigate the strain in the country.