Chinese doctors have reported that human-to-human transmission likely occurred in a small family cluster of H5N1 avian flu cases in China late last year.

A 52-year-old man from Jiangsu province fell ill with the virus after helping care for his son, 24. The younger man died from his infection Dec. 2 but the father recovered.

In an article published electronically by the British journal The Lancet on Tuesday, Chinese doctors reported that molecular analysis showed that viruses from the two men were virtually identical. They were fully avian viruses, meaning they hadn't swapped genes with any human flu viruses or viruses from another mammal.

The similarity of the viruses and the investigation into the possible sources of infection for the two men point to limited person-to-person spread, the authors said.

"We believe that the index case (the son) transmitted H5N1 virus to his father while his father cared for him in the hospital," they wrote.

The son, who died before doctors would query him about how he might have become infected, may have been exposed at a live animal market he visited six days before he fell ill, the article suggested.

The virus taken from the father included one small mutation on one of the internal genes. That change is not thought to be one that gives the virus greater ability to infect humans or to transmit human to human.

"Always if you isolate two viruses, they are going to have some changes," said Dr. Adolfo Garcia-Sastre, an influenza expert at New York's Mount Sinai Hospital.

"It doesn't mean that they are adaptive changes.… The important thing is the opposite thing. They are so close that then it's clear that that is the same virus."

Human-to-human transmission reported in other countries

Dead-end clusters of cases like this one have been reported in a number of countries, including Hong Kong, Thailand, Indonesia and Pakistan.

Isolated cases of human-to-human transmission do not indicate that the H5N1 virus is poised to begin spreading more rapidly. In fact, this chain of transmission died out four months ago, when the father recovered without spreading the disease to any of his caregivers or contacts.

"It doesn't mean anything that we haven't known about since the human-to-human transmission was well documented in Thailand a couple of years ago," said Dr. Arnold Monto, an influenza expert at the University of Michigan.

But Monto, echoing a commentary piece also published by the journal, said each of these cases provides an opportunity for the virus to mutate in ways that would make it more transmissible to and among people.

"As long as we continue to have avian outbreaks we're going to see occasional human cases and unsustained human-to-human transmission," he said.

"Now the concern is that if that if this keeps happening, the worrisome mutations may take place. But there is no guarantee one way or the other that they will take place."

The article also revealed interesting information about the novel treatment used in the care of the father.

Doctors gave the man two transfusions of blood from a woman who had been vaccinated in a clinical trial of Chinese vaccine maker Sinovac's H5N1 avian flu vaccine.

Interestingly, the virus that vaccine was made to protect against was not a perfect match to the one that infected the man and his son. They were infected with a virus from the family or clade known as 2.3.4, which is currently circulating in parts of Asia. The vaccine was made with a clade 1 virus recovered in Vietnam in 2004.

Vaccine makers are hoping H5N1 vaccines will offer some cross protection against mutated strains. If they do, it would strengthen their arguments that governments should stockpile supplies of H5N1 vaccine to be used in the early days of a pandemic, if H5N1 goes on to cause a pandemic.

As well, some experts have questioned whether blood from recovered cases — or perhaps vaccinated people — might be useful as a therapy during a pandemic, when drugs and vaccines are expected to be in short supply. The idea is that the antibody built up by the donor would help the recipient fight off the infection.