All of the contacts of a 2-year-old boy who tested positive for the H5N1 strain of avian influenza in Hong Kong and the Chinese mainland were either asymptomatic or tested negative for bird flu, authorities of the special administrative region and Guangdong province said, suggesting that the chance of an outbreak was slim.

The boy was still in serious condition after a week of treatment in Hong Kong.

He is being kept isolated in the pediatric intensive care unit at Princess Margaret Hospital.

It was the city's first human case of bird flu in 18 months. The boy is thought to have been infected with the deadly virus during his visit to a wet market in Guangzhou last month.

Hong Kong kept the bird flu alert level at "serious" after raising it on Friday, and stricter visiting restrictions were imposed in the city's public hospitals.

Imports and sales of poultry have continued as usual - unlike in December, when an infected live chicken was found. Samples collected from 30 local chicken farms on Sunday all tested negative for bird flu, a spokesperson from the city's agriculture, fisheries and conservation department said.

York Chow Yat-ngok, secretary for food and health, assured the public that it was an "isolated" case and the city is well guarded against all types of infectious diseases.

"We feel that there is no need for panic among Hong Kong residents," he said. "All in all, I would only advise the public to maintain a good level of personal and environmental hygiene."

Though summer is not considered a peak season for flu, Chow said human flu cases are not seasonal in nature, as observed in tropical countries, like Vietnam and Indonesia.

The boy's illness was first reported before midnight on Friday. His condition was said to be stable, but it worsened to serious on Saturday.

The boy, who lives with his parents and maternal grandmother in the Haizhu district of Guangzhou, had visited a wet market on Nanyuan Street in mid May where his mother had bought a live duck.

Returning home from a three-day tour to Anhui province, the boy developed a fever and runny nose on May 23. He was taken to a private medical clinic in Hong Kong three days later.

He was taken to a hospital emergency room on May 28 and was thought to have encephalitis. However, he tested positive for the H5N1 strain of avian influenza on Friday.

All of the boy's 80 contacts in Hong Kong were either asymptomatic or tested negative for bird flu leading the center for health protection to suggest that the chance of human-to-human transmission was slim.

As a precaution, however, the center alerted doctors on Saturday that special attention should be paid to patients who had contact with poultry in Guangzhou.

No one in Guangzhou who had close contact with the boy showed any sign of infection on Monday and the source of the virus had yet to be determined, local authorities said.

Among those tested were poultry venders at the wet market in Haizhu district, said He Jianfeng, director of the Epidemic Studies Institute of the Guangdong Disease Control and Prevention Center.

The source of the infection may not be determined because the duck had been eaten and too much time has passed since the boy was infected, He said.

"Vigilance has been maintained against a human avian flu epidemic in Guangdong. No sample has tested positive so far this year. The boy infected with avian flu is apparently an isolated case," He said.

He advised that people stay away from live poultry, cook poultry thoroughly and wash their hands after handling poultry.