By Ho Binh Minh

HANOI, April 14 (Reuters) - Two people in northern Vietnam have been infected with the H5N1 bird flu virus and health authorities are carrying out tests to see if 11 others who have come down with fever have been infected.

The two who tested positive -- a 22-year-old man and a 27-month-old baby girl -- as well as the 11 suspected cases are all residents in the same commune in Bac Kan province.

"All the patients have been isolated and tests were taken to verify the reason of their sickness," said Deputy Director Luu Xuan Hoa of the Cho Moi Medical Centre in Bac Kan province.

"She (the baby) is a neighbour of the first man who has tested positive for the H5N1 virus, while others live in the same hamlet," he said.

Four people on the suspect list have been discharged.

Disease clusters are a special concern because it may mean the agent - in this case H5N1, which kills up to 60 percent of those it infects - is gaining the ability to jump from person to person.

Vietnamese media, however, reported that dead poultry had been seen in the village and that the patients had either eaten or come in contact with sick birds. This opens the possibility that the two patients may have been infected by the same source.

"We have no evidence to conclude that there is a human-to-human infection," Hoa said.

Vietnam had two bird flu deaths earlier this year - a 38-year-old woman and a three-year-old girl.

The H5N1 virus has once been a disease confined largely to birds, only rarely infecting people.

Since 2003, it has infected 493 people, killing 292, or nearly 60 percent, with Vietnam reporting 59 deaths. Almost all these infections were believed to have taken place directly from bird to human.

There have been two sizeable clusters - one in which eight family members died on the Indonesian island of Sumatra in 2006 and another in Turkey in which eight people were infected and four died.

In the Sumatra case, the virus went on for two generations and then stopped - a 37-year-old woman was believed to have infected her 10-year-old nephew, who went on to infect his father.

Another smaller probable case of human-to-human transmission occurred in Thailand in 2004, where a mother died after tending to her sick daughter for hours. (Additional reporting by Tan Ee Lyn in Hong Kong; Editing by Ron Popeski)