Atlanta - U.S. health officials said on Thursday that it was unlikely that a deadly "bird flu" virus in Hong Kong has been transmitted from person to person, but they still cannot rule out the possibility.
The Centers for Disease Control and Prevention (CDC) said preliminary results from its investigation of Hong Kong's 16 confirmed and three suspected human infections suggests that the avian influenza A virus is "not being efficiently transmitted to humans at this time."
"We can't rule out that it's happening, but if it is happening, the data that we have right now indicates that it's not happening very efficiently, and that's certainly good news," CDC spokesman Tom Skinner said.
Four of the people who became infected have died and three others remain in critical condition, the CDC said.
The CDC said most infections with the influenza A(H5N1) virus appear to have been caused by exposure to infected poultry or direct exposure to the virus in the laboratory.
"However, the investigation has not ruled out the possibility of person-to-person transmission from exposure to ill and infected persons," the CDC said.
Two people who came in contact with the first-ever human case, a three-year-old boy who died of respiratory failure in May, later became infected.
One of them, a health-care worker, reported no recent exposure to poultry or any history of exposure to the virus in the laboratory.
"He developed antibodies and we know that he actually saw the patient that had this," Skinner said. "We're not able to determine if, in fact, he caught it directly from this person."
The other, a child who attended the same day-care center as the first victim, may have been infected by the sick child or by the same chickens that apparently infected the three-year-old, the CDC said.
However, members of all but one of the victims' families have not become infected with the virus, leading researchers to conclude that person-to-person transmission is unlikely.
"Person-to-person transmission is not going on at a very efficient rate," Skinner said.
Skinner said it was "encouraging" that no new cases of H5N1 infection in humans appear to have occurred since Dec. 28.
Two groups of slightly different viruses have been found among the cases of human infection which have occurred throughout Hong Kong. Researchers said they have not seen any evidence of genetic mutations that might turn the "bird flu" into a more easily transmitted human strain.
Hong Kong slaughtered 1.4 million chickens and other poultry this month in an attempt to contain the disease. Live chicken importation from China, which supplied much of the territory's needs, was banned.
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