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Update: Isolation of Avian Influenza A(H5N1) Viruses from Humans -- Hong Kong, 1997-1998

As of January 6, 1998, a total of 16 confirmed and three suspected cases of human infection with avian influenza A(H5N1) viruses have been identified in Hong Kong. Confirmed cases are those from which an influenza A(H5N1) virus was isolated or in which a seroconversion to influenza A(H5N1) virus was detected by a neutralization assay. Suspected cases are those with influenza-like illness (ILI) and preliminary laboratory evidence of influenza A(H5N1) infection. This report summarizes interim findings from the ongoing epidemiologic and laboratory investigation of influenza A(H5N1) cases by health officials in Hong Kong and by CDC.

The first known case of human infection with influenza A(H5N1) occurred in a 3-year-old boy who died from respiratory failure in May 1997 (1). Of the 15 remaining confirmed cases, five persons had onset of illness in November and 10 in December; all three persons with suspected cases had onset during December. No cases have been identified with onset after December 28, 1997. Ages of persons with confirmed cases ranged from 1 to 60 years (mean age: 17 years) and, for persons with suspected cases, from 3 to 7 years (mean age: 5 years). Nine (47%) cases occurred among persons aged less than or equal to 5 years. Four persons with confirmed cases have died, and three remain in critical condition.

Testing has been completed of serum samples collected in August as a part of the epidemiologic investigation of the first case of human influenza A(H5N1) infection. Serum samples were obtained from 502 persons who may have had contact with the child or with poultry, including family members, persons who lived in the same neighborhood, children and staff of the child-care center the child attended, health-care workers, poultry workers, and persons working on pig farms. Samples of control serum specimens were obtained from 218 healthy children and 201 healthy adult residents of Hong Kong. These samples were tested for antibody to influenza A(H5N1) virus using a micro-neutralization assay. Of the 502 persons tested who may have had contact with the child or with poultry, elevated neutralization antibody titers to influenza A(H5N1) virus were present in nine (2%). These persons included five (17%) of 29 poultry workers, one (2%) of 54 health-care workers, one (2%) of 63 neighbors, one (1%) of 73 laboratory workers, and one (0.4%) of 261 child-care center contacts. Specimens were negative for the four family members, 18 persons working on pig farms, and the 419 controls. Seropositivity was not associated with reported ILI.

Antigenic and genetic analyses of viral isolates from seven case-patients indicated two closely related but distinguishable groups of influenza A(H5N1) viruses, suggesting multiple introductions in humans from poultry sources. All seven of the influenza A(H5N1) viruses analyzed from human cases contained all eight RNA gene segments from avian viruses, indicating that genetic reassortment between avian and human influenza viruses has not occurred.

Reported by: TA Saw, FHKAM (Community Medicine), Hong Kong Dept of Health; W Lim, FRCP, Virus Unit, Hong Kong Dept of Health; K Shortridge, PhD, The Univ of Hong Kong; J Tam, PhD, Chinese Univ of Hong Kong; KK Liu, DRVS, Dept of Agriculture and Fisheries; KH Mak, FHKAM (Community Medicine); T Tsang, MPH, TK Au, MPH, YY Ho, MSC, TY Lee, MBBS, H Kwong, MMED (Public Health), Hong Kong Dept of Health. Prince of Wales Hospital; Princess Margaret Hospital; Queen Elizabeth Hospital; Queen Mary Hospital; Tuen Mun Hospital; United Christian Hospital; Yan Chai Hospital, Hong Kong. RG Webster, PhD, St. Jude Children's Research Hospital, Memphis, Tennessee. World Health Organization, Geneva, Switzerland. Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The cases reported in Hong Kong represent the first identified instances of human illness associated with infection with influenza A(H5N1) viruses. Goals of the ongoing investigation are to detect new cases, determine sources of infection and mode(s) of transmission, and identify risk factors for influenza A(H5N1) infection. Except for a cluster of two confirmed and two suspected cases in one family, case-patients are not known to have had contact with each other or a common source of exposure and are geographically distributed throughout Hong Kong. All cases of infection have occurred among residents of Hong Kong, and no cases of infection with influenza A(H5N1) viruses have been identified among persons residing outside Hong Kong.

The serologic data obtained as part of the epidemiologic study of the initial case support the preliminary conclusion that persons with high levels of exposure to infected poultry or direct exposure to the virus in the laboratory may be at increased risk for infection with influenza A(H5N1) virus. However, the investigation has not ruled out the possibility of person-to-person transmission from exposure to ill and infectious persons: two seropositive persons who had contact with the first case-patient included a child-care center classmate and a health-care worker, and the classmate had contact with both the ill child and the same potential environmental source of exposure to ill chickens at the school as the ill child. However, the health-care worker reported no history of exposure to the virus in the laboratory or any recent exposure to poultry, and a history of exposure to the child or to poultry was unknown for a seropositive elderly neighbor. On the basis of the overall low rates of infection among contacts and controls and the lack of seropositivity among family members, at this time, the virus probably is not being efficiently transmitted among humans.

Global surveillance for influenza viruses is critical to monitor the circulation of different strains and indicates that human influenza type A(H3N2), type A(H1N1), and type B viruses continue to circulate worldwide. Data from the Hong Kong Department of Health's influenza surveillance system indicate that the number of cases of ILI in Hong Kong is at normal levels for this period; however, during December, the number of human influenza viruses isolated increased. During December, influenza A(H3N2) was the most commonly isolated influenza strain in Hong Kong, although influenza A(H1N1) and B viruses also were identified. The currently available inactivated trivalent influenza vaccine contains influenza A(H1N1), A(H3N2), and B strains representative of those currently circulating among humans and is recommended for persons at increased risk for influenza-related complications (2).

Information about influenza A(H5N1) activity in Hong Kong and the United States and international influenza surveillance data are available through CDC's Influenza Branch, Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases, World-Wide Web site


  1. CDC. Isolation of avian influenza A(H5N1) viruses from humans -- Hong Kong, May-December 1997. MMWR 1997;46:1204-7.

  2. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1997;46(no. RR-9).

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