The strain known as H7N9 was first reported by Chinese authorities in March. As of the end of May, there were 132 cases and 37 deaths in China and Taiwan linked to the virus.
Health officials suspect patients were most likely infected by birds in live animal markets but acknowledged sporadic cases of the virus probably were spreading among humans. Except for a single case reported last month, the infections appear to have stopped since Chinese authorities took measures to slow the virus, including shutting down live markets.
In the new study, Chinese researchers interviewed the family and close friends of a father and daughter both killed by H7N9 in eastern China to figure out how the virus might have spread between them. Both patients lived in the same household, were critically ill during the investigation and could not be interviewed.
The father, 60, was in charge of buying food for the family and bought six live quails before falling sick. His daughter, 32, rarely left the residential district where they lived and didn't have any known contact with birds, except for two black swans raised by the property owners.
The daughter took care of her father when he became ill, without wearing any protective equipment. She fell sick several days afterward and died one month later. The bird flu viruses isolated from the father and daughter were nearly genetically identical.
There is no definitive test to prove when a virus has spread from human-to-human, but scientists consider matching viruses and eliminating other ways the virus might have spread to be convincing evidence. Scientists also tested 43 contacts of the two patients; none had H7N9.
"In this cluster, the virus was able to transmit from person-to-person," wrote Xian Qi of the Jiangsu Province Center for Disease Control and Prevention, who was the lead author of the study. The scientists concluded the transmission was "limited and non-sustainable." The paper was published online Tuesday in the journal BMJ.
"It is also notable that the transmission occurred between blood relatives," said Dr. Peter Horby, a bird flu expert at Oxford University Clinical Research Unit in Hanoi, Vietnam, in a statement. Horby, who was not involved in the latest study, noted there is some evidence that genetic factors may make some people more susceptible to bird flu.
In an accompanying commentary in the BMJ, experts said similar patterns had been seen with other types of bird flu, including H5N1, another feared bird flu strain that first emerged in 1996 and has since killed millions of chickens. It has sickened more than 600 people and caused 377 deaths, mostly in Asia.
"To observe some transmission of H7N9 from human-to-human...does not necessarily indicate the virus is on course" to spark a pandemic, wrote James Rudge of the London School of Hygiene and Tropical Medicine, who is based at Mahidol University in Thailand, and Richard Coker of the National University of Singapore.
Still, Rudge and Coker noted several worrying features about H7N9, such as its ability go undetected in birds before sickening humans. They also warned officials to be on guard for a possible return of H7N9 in the winter; flu viruses typically spread more easily in cold weather.
"The threat posed by H7N9 has by no means passed," they said.