Health authorities confirmed Thursday that a 52-year-old man from Jiangsu province, who became the country's latest human case of bird flu, contracted the H5N1 virus from his son.
However, no mutation of the virus has been detected.
Mao Qun'an, spokesman for the Ministry of Health, said the initial conclusion reached by experts was that the father-son case was "an infection through close contact".
"But there were no biological features for human-to-human transmission," he told a regular press conference.
The virus remained un-mutated as the type that mainly affects birds, rather than humans, according to experts.
The World Health Organization (WHO) has warned that the virus - if given sufficient opportunity - could mutate into a form that is highly infectious and easily transmitted from person to person, which could start a global pandemic.
However, this case - although it involved the disease apparently passing from one person to another - did not exactly fit the profile of an infectious human-to-human outbreak, Mao said.
The case puzzled experts.
Dr Hans Troedsson, the WHO's representative in China told China Daily: "The virus can still not be transmitted from human to human efficiently."
Mao said that none of the 82 people known to have had close contact with the two infected men had contracted the virus.
He dismissed fears of a possible mutation last month when the father surnamed Lu, who has since recovered, was hospitalized with the deadly H5N1 strain of the virus soon after his 24-year-old son died from it on December 2.
The cause of the initial infection of the son, however, remains unclear, as neither man had had any contact with sick or dead birds - the primary known source of human cases, Mao said.
However, the possibility of animal-to-human infection cannot be excluded at this stage, Troedsson said.
The WHO has offered technical support to the ministry.
The father-son case, although unusual, was not the only one of its kind to occur in China, Mao said, declining to give more details.
A similar case occurred in Pakistan last month.
The latest cases bring the number of confirmed human infections of bird flu in China to 27 since 2003, with 17 deaths.
Bird flu, or avian influenza, is a contagious disease of animal origin caused by viruses that normally infect only birds and less commonly, pigs.
The Ministry of Agriculture said early last month that the possibility of regional bird flu outbreaks were "very high" in the winter and spring.
An outbreak of bird flu was reported in Xinjiang Uygur autonomous region late last month, leading to the culling of about 30,000 poultry.
Local authorities said the outbreak was already put under control.