Past Examples of Probable Limited, Non-Sustained, Person-to-Person Spread of Avian Influenza A Viruses

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Avian influenza A viruses infect the respiratory and gastrointestinal tracts of birds but rarely infect people. Most previous avian influenza A virus infections of people have occurred following close, prolonged, and unprotected (e.g., no gloves or medical mask) contact with infected poultry or environments contaminated by their saliva, mucous, or feces that can contain these viruses. The risk of human infection depends, in part, on the duration of exposure and the specific exposures encountered.

Because human infection with avian influenza A virus poses pandemic potential, every case must be investigated to assess whether human-to-human transmission might have occurred. Detailed public health investigations can help determine whether person-to-person spread of an avian influenza A virus occurred. In some clusters of epidemiologically related cases of avian influenza A virus infection, limited, non-sustained, human-to-human transmission is thought to have occurred to persons without poultry exposures. No instances of human-to-human transmission of avian influenza A viruses have been reported in the United States. Below are examples of probable limited, non-sustained, human-to-human transmission of different avian influenza A viruses in other countries.

Examples of probable person-to-person spread of avian influenza A viruses:

  • In 1997, the first human infections with highly pathogenic avian influenza A(H5N1) virus were identified in Hong Kong. Serologic studies suggested evidence of probable limited, non-sustained, human-to-human H5N1 virus spread.
  • In 2003, in the Netherlands, there was evidence of limited, non-sustained spread of highly pathogenic avian influenza A(H7N7) virus from two poultry workers to three family members. All three family members had conjunctivitis (eye inflammation) and one also had influenza-like illness after exposure to an ill poultry worker at home.
  • In 2004, in Thailand, there was evidence of limited, non-sustained, person-to-person spread of highly pathogenic avian influenza A(H5N1) virus in family members. Spread of H5N1 virus was associated with prolonged, very close, unprotected contact between an ill child with H5N1 virus infection and her mother and aunt while the child was hospitalized.
  • In 2005, in Indonesia, limited, non-sustained, person-to-person spread of highly pathogenic avian influenza A(H5N1) virus in households among family members could not be excluded among two groups of cases who had no known contact with poultry or other animals but who did have prolonged, unprotected contact with a symptomatic person with H5N1 virus infection.
  • In 2006, in Indonesia, limited, non-sustained, person-to-person-to-person (3rd generation) spread of highly pathogenic avian influenza A(H5N1) virus may have occurred among a family cluster of eight probable or confirmed H5N1 cases.