Editor"e;s note: This story was updated Jan 14 with additional information from a statement by the WHO"e;s Western Pacific office.

Jan 13, 2004 (CIDRAP News) – The World Health Organization (WHO) confirmed today that an avian influenza virus has infected humans in Vietnam, raising concern that avian and human viruses could share genes and give rise to a dangerous new influenza virus.

The WHO said viruses isolated from two children and one adult in Hanoi who had a severe respiratory illness were identified as avian influenza virus A(H5N1). The three patients later died, officials said. Since the end of October, 13 children and one adult from Hanoi and surrounding areas have been hospitalized with severe respiratory illness, which proved fatal for the adult and 11 of the children, the agency said.

"It is not known if all of these cases were caused by the same pathogen," the WHO said in a statement posted on the agency"e;s main Web site. "At present, there is no evidence that human-to-human transmission has occurred. No reports indicate that health care workers have been infected."

Some of the cases have involved family members, but the cases may have resulted from exposure to the same source in poultry rather than from spread within the families, according to a statement from the WHO"e;s Western Pacific Region office. "WHO would view the presence of bird flu in humans as serious if human-to-human transmission had occurred," the statement said.

An outbreak of highly pathogenic avian influenza was reported last week in two provinces in southern Vietnam. The WHO said today that the H5N1 virus was identified as the cause. Official sources said the outbreak killed 40,000 chickens, but other reports said as many as 800,000 birds were affected, including some near Hanoi in northern Vietnam.

In addition, South Korea had an outbreak of H5N1 avian influenza in several provinces in December, and Japanese authorities yesterday reported the death of 6,000 chickens at one farm from influenza. The WHO said the Japanese outbreak also involved the same virus. No human cases were reported in those outbreaks.

"The circulation of highly pathogenic avian influenza viruses in large numbers of poultry in a growing number of countries is of particular concern," the WHO said. "Influenza viruses are highly unstable. The co-circulation of highly pathogenic animal viruses with human viruses could create opportunities for different species-specific viruses to exchange genetic material, giving rise to a new influenza virus to which humans would have little, if any, protective immunity."

The first known cases of human illness from an H5N1 virus occurred in Hong Kong in 1997, when 18 people fell ill and 6 of them died, the WHO noted. The immediate killing of 1.5 million chickens in Hong Kong probably prevented a larger human outbreak, the agency said.

Other recent outbreaks of human illness from avian flu viruses have been smaller or less severe, the WHO said. The H5N1 virus caused two human cases, with one death, in Hong Kong in February 2003. Last year in the Netherlands, H7N7 avian influenza caused the death of a veterinarian and mild illness in 83 other people. In Hong Kong, mild infections with influenza A(H9N2) occurred in two children in 1999 and in another child in December 2003.

The WHO said Hong Kong"e;s National Influenza Center identified the H5N1 virus in samples from three of the Vietnamese patients. Samples have also been sent to Japan"e;s National Institute for Infectious Diseases for analysis. Both laboratories are members of WHO"e;s Global Influenza Surveillance Network.

"The relationship between the human and poultry outbreaks of avian influenza A(H5N1) is Viet Nam is not fully understood at present," the WHO stated. The agency and Vietnam"e;s Ministry of Health are trying to identify the source of the human cases and determine whether human-to-human transmission has occurred, officials said.

Flu pandemics in the 20th century are believed to have arisen from avian flu viruses, although the avian viruses first jumped to other animals and were genetically altered before they spread widely in humans (see link to CIDRAP information below).