A study published on the website of the US journal Science on 18 July said the new strain of bird flu virus is highly transmissible among ferrets, a widely used animal model for studying how flu might spread in humans.

Consequently, it is possible the virus could efficiently spread among humans eventually, posing a pandemic risk, according to the study led by Chen Hualan, director of China's National Avian Influenza Reference Laboratory at Harbin Veterinary Research Institute.

Zeng Guang, chief epidemiologist at the Chinese Center for Disease Control and Prevention, disagrees with that conclusion.

"The findings are mainly based on animal tests in the lab that have not been witnessed or substantiated among the H7N9 human cases reported. So it shouldn't affect current intervention efforts or strategy at all," he said.

"So far, no substantial evidence of H7N9 spreading among humans has been detected," he said.

The study showed one virus isolated from humans was able to transmit efficiently among ferrets through respiratory droplets, raising the possibility of eventual airborne transmission among humans.

The study also warned that the nonpathogenic nature of the H7N9 virus in poultry enables it to "replicate silently" in avian species and be transmitted to humans, providing further opportunities for the virus to acquire more mutations and become more virulent and transmissible in the human population.

To date, the H7N9 virus has caused 133 human infections with 43 deaths on the mainland since February, statistics from the National Health and Family Planning Commission showed.

During summer, the mobility of H7N9 stayed relatively weak, but sporadic human cases could be expected, Dr Guang said.

He referred to a newly confirmed case reported in Beijing on Saturday. The patient, a 61-year-old woman from Hebei province, is in critical situation and being treated at Beijing Chaoyang Hospital.

"We have to carefully watch the epidemic situation in the coming autumn, when the virus mobility tends to increase," Dr Guang said.

To investigate possible origins of the new virus infecting both birds and humans, Mr Hualan's team collected more than 10,000 samples from poultry markets, poultry farms, wild bird habitats and poultry and swine slaughterhouses across China from March to May.

"Precautionary policies and measures for possible re-emergence of the H7N9 virus in the future have to be well prepared, otherwise it might hit people hard," Mr Hualan said.