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A late May World Health Organization (WHO) update has concluded that a family cluster of avian influenza virus H5N1 infections did not represent any major changes in the virus or in its ability to infect humans.1


The outbreak, in a village on the island of Sumatra in Indonesia, is the largest known cluster of human cases since H5N1 influenza first reappeared in Asia in 2003. These were not the first cases of presumed human-to-human transmission of the virus,2, 3 but the size of the cluster concerned health officials.


The presumed index patient, a 37-year-old woman, died before being tested for H5N1. Subsequently, there were seven confirmed H5N1 infections in family members, six of whom died. All were blood relatives; spouses were not infected, further supporting the theory that some people may have a greater genetic susceptibility to the virus than others.4


As in earlier suspected cases of human-to-human transmission of the virus, all of the confirmed cases were directly linked to close, prolonged, and unprotected exposure to the original case while she was acutely ill and coughing.1 At press time, intense follow-up by the WHO and the Indonesian Ministry of Health had uncovered no further transmission of infection to either health care workers or to the larger community. Fifty-four surviving family members and other close contacts of cases were identified and placed under voluntary home quarantine, and most were receiving prophylactic oseltamivir.5


Genetic studies of these H5N1 isolates confirmed that all seven were related,6 indicating that the infection was shared among family members and not coincidentally acquired from other sources. The strain was entirely avian, without evidence of genetic reassortment involving human or pig influenza viruses.1 These studies confirmed that H5N1, like other influenza A viruses, continues to evolve. However, expert opinion continues to hold that the virus has not yet progressed to a form that can more easily infect humans.7


A large family or community cluster presents the kind of scenario that could be expected at the start of a pandemic. Because there is no evidence in this cluster of either significant mutations in the virus itself or sustained human-to-human transmission, the WHO's pandemic alert level remains at phase 3.5




1. World Health Organization. Avian influenza-situation in Indonesia-update 14. 2006 May 23. [Context Link]


2. Hayden F, Croisier A. Transmission of avian influenza viruses to and between humans. J Infect Dis 2005;192(8):1311-4. [Context Link]


3. Ungchusak K, et al. Probable person-to-person transmission of avian influenza A (H5N1). N Engl J Med 2005;352(4):333-40. [Context Link]


4. Beigel, JH, et al., for The Writing Committee of the World Health Organization (WHO) Consultation on Human Influenza A/H5. Avian Influenza A (H5N1) Infection in Humans. N Engl J Med 2005;353(13):1374-85. [Context Link]


5. World Health Organization. Avian influenza-situation in Indonesia-update 16. 2006 May 31. [Context Link]


6. Rosenthal E. Human-to-Human Infection by Bird Flu Virus Is Confirmed. New York Times 2006 Jun 24. A8. [Context Link]


7. European Centre for Disease Prevention and Control. Technical report: ECDC scientific advice: the public health risk from highly pathogenic avian influenza viruses emerging in Europe with specific reference to type A/H5N1; 2006 Jun 1. [Context Link]