A 9-year-old boy in Vietnam has died of bird flu (search), bringing the number of people in the country killed by the virus to 21, a doctor said Wednesday.

The boy from the southern Mekong Delta province of Tra Vinh was admitted to the Hospital for Tropical Diseases (search) in Ho Chi Minh City early Tuesday morning and died later that day, hospital deputy director Tran Tinh Hien said.

"He was tested positive for H5N1 virus (search)," Hien said, refering to the strain of the virus that is deadly to humans.

World health experts fear that bird flu might mutate and create the next influenza pandemic. So far, there has been no concrete evidence of human-to-human transmission of bird flu. Only Vietnam and Thailand have recorded human deaths from the virus.

The boy who died Tuesday ate a sick chicken last week and developed a high fever and serious lung infection, said Le Hai Au, director of the Tra Vinh provincial Preventive Medicine Center.

The boy had "all the clinical symptoms of bird flu," Au said

None of the boy's family members has similar symptoms, he said.

The hospital was also treating a 16-year-old girl from Dong Thap province who tested positive for the H5N1 strain of bird flu.

The girl was in critical condition and has been breathing with a respirator since being admitted to the hospital last week, Hien said.

Vietnam has reported bird flu outbreaks in several provinces in recent months. Before Wednesday, its last human fatality was in early September.

Cooler temperatures and an increase in the movement of poultry ahead of the Lunar New Year in early February could help spread the virus, animal health officials have warned.