The development was "worrying" and should be closely watched, the team wrote in the British online journal bmj.com, but stressed that the virus, believed to jump from birds to people, was still inadept at spreading among humans.
"People should not panic," epidemiologist Chang-jun Bao of the hard-hit Jiangsu province's Center for Disease Control and Prevention, told AFP of the report that he co-authored.
"The transmissibility of this novel virus... was not so effective."
Scientists have long feared the virus would mutate into a form that transmits easily from person to person.
In the new study, Bao and a team report on the case of a 60-year-old man who died in hospital after contracting the H7N9 virus, which he apparently transmitted to his daughter.
The 32-year-old woman, who had nursed her father for over a week, also died in hospital.
She had had no access to potentially infected poultry, leading investigators to conclude that the "most likely explanation" for her illness was direct virus transmission from her father, who had regularly visited live poultry markets.
"Aside from the exposure to her father's respiratory secretions during her bedside care, the daughter had no definite exposure to poultry or other suspect sources of infection," said Bao.
Genetic tests of virus samples from the two patients also revealed they were "almost identical".
Despite this apparent evidence of direct transmission, the virus's transmissibility remained "limited and non-sustainable," said the paper. None of 43 other people who had had close contact with the two patients, including hospital staff, contracted the virus.
"These findings suggest that potential genetic susceptibility might be one of the determinants and that avian influenza viruses... are more easily transmitted between individuals with genetic connection," said the paper.
The scientists noted they had been unable to interview the two critically ill patients and could not definitively rule out the possibility that the daughter picked up the virus somewhere else, although this seemed "less likely".
Official figures released last month said the H7N9 virus had made 132 people ill in mainland China since the first human cases were reported in March, of which 43 died. One case was recorded in Taiwan.
A study in June warned that a lull in new infections may lift in the autumn.
The results of a lab study published in the US journal Science in May showed the H7N9 strain can spread among mammals, specifically ferrets, and could do the same between humans under certain conditions.
H7 influenza viruses comprise a group that normally circulate among birds, of which H7N9 forms a subgroup that had never been found in humans until the Chinese outbreak.
Commenting on the paper in a BMJ editorial, researchers James Rudge and Richard Coker of the Communicable Diseases Policy Research Group in Thailand said it provided the "strongest evidence yet" of H7N9 transmission between humans.
"Does this imply that H7N9 has come one step closer towards adapting fully to humans? Probably not," they wrote.
Odd cases of human-to-human transmission had also been reported in other bird flu types like H5N1 and H7N7 -- none of which has mutated into an easily spreadable form, and so to see it in H7N9 was not surprising.
"While the paper... might not suggest that H7N9 is any closer to delivering the next pandemic, it does provide a timely reminder of the need to remain extremely vigilant: the threat posed by H7N9 has by no means passed," said Rudge and Coker.
Peter Horby, senior fellow at Oxford University's Clinical Research Unit in Hanoi, Vietnam, agreed that limited human-to-human transmission "does not necessarily represent the early stages of a trajectory towards full adaptation to humans".