HONG KONG, March 24 -- Center for Health Protection Controller Dr. Thomas Tsang said here Saturday they have ruled out the possibility of human-to-human transmission after tests on a nine-month baby's recent infection of H9N2, a mild form of avian influenza.

Speaking on a radio talk show on Saturday, Dr. Tsang said genetic sequencing studies of the virus isolated from the nine-month-old girl recently hospitalized for H9N2 showed all genes from the virus are of avian origin.

Dr. Tsang said the results suggested the virus was directly transmitted from a bird to the girl, without mingling with a human flu virus.

As the girl did not contact wild birds before the onset of the symptoms, Dr. Tsang said she may have contracted the virus at the Tseung Kwan O market in Hong Kong's New Territories she visited several times with her family.

Noting tests on respiratory specimens taken from a healthcare worker and three children who were in the same cubicle in the United Christian Hospital with the girl in early March were all negative for the H9 virus and her family members have no symptoms, Dr. Tsang said the case did not involve human-to-human transmission.

However, he said children with poorer immunity may be more prone to the virus as H9N2 cases recorded in 1999 and 2003 also involved children aged below five.

On average samples of 40,000 flu cases will be tested a year for surveillance, Dr. Tsang said both H5 and H9 viruses are related to birds and can be transmitted from birds to humans, but the former has a mortality rate of 70 percent while the latter is a mild form of avian flu and its symptoms are similar to common colds and flu.

When asked whether global warming will lead to virus mutation, Dr. Tsang said there is no direct link between the weather and virus reassortment, but warm weather will favor the breeding of germs and communicable disease vectors.