Examples of Human Infections with Avian Influenza A Viruses with Possible Limited, Non-Sustained Human-to-Human Transmission

Human infections with avian influenza A viruses are rare and have most often occurred after people had exposure to infected poultry (e.g. direct contact with chickens, or visiting a live poultry market). However, some clusters in which limited, non-sustained person-to-person spread of avian influenza A viruses was suspected or is believed to have occurred have been reported in several countries. There is no test to confirm human-to-human spread of avian influenza A viruses. Rather, a determination that human-to-human transmission likely occurred is based upon the findings of detailed epidemiologic and laboratory investigations.

For example:

• In 1997, when the first human cases of Asian H5N1 virus infection were identified in Hong Kong, there was serologic evidence of limited, non-sustained transmission of Asian H5N1 virus to a very small number of health care workers and household contacts, but the virus did not spread further. (Buxton Bridges C et al. “Risk of influenza A (H5N1) infection among health care workers exposed to patients with influenza A (H5N1), Hong Kong;” Katz JM et al. “Antibody response in individuals infected with avian influenza A (H5N1) viruses and detection of anti-H5 antibody among household and social contacts.”)
• In 2003, in the Netherlands, there was evidence of possible transmission of H7N7 virus from two poultry workers to three family members. All three family members had conjunctivitis and one also had influenza-like illness. (M Du Ry van Beest Holle, Meijer, et al, 2005. “Human to Human Transmission of Avian Influenza /H7N7, The Netherlands, 2004.”)
• In 2004, in Thailand, there was evidence of probable human-to-human spread of Asian H5N1 virus in a family cluster. Transmission was associated with prolonged very close unprotected contact between an ill child with H5N1 virus infection and her mother and her aunt. Further transmission did not occur. (Ungchusak et al, 2005. “Probable Person-to-Person Transmission of Avian Influenza A (H5N1).”)
• In 2005, in Indonesia, limited, non-sustained person-to-person transmission of Asian H5N1 virus could not be excluded among two clusters of patients who had no known contact with poultry or other animals. (Kandun et al, 2006. “Three Indonesian Clusters of H5N1 Virus Infection in 2005.”)
• In 2006, in Indonesia, limited, non-sustained person-to-person transmission of Asian H5N1 virus may have occurred among a family cluster of 8 probable or confirmed Asian H5N1 cases. (Avian influenza – situation in Indonesia – update 16)
• In December 2007, in China, limited, non-sustained Asian H5N1 virus transmission is thought to have occurred between a sick son and his father through prolonged very close unprotected exposure while the son was in the hospital. (Wang et al, 2008. “Probable limited person-to-person transmission of highly pathogenic avian influenza A (H5N1) virus in China.”)
• Also in 2007, in Pakistan, limited, non-sustained human-to-human H5N1 virus transmission is thought to have occurred among brothers. (WHO, Weekly Epidemiological Record, 2008. Human cases of avian influenza A(H5N1) in North-West Frontier Province, Pakistan, October–November 2007
• In 2013, human infections with an Asian lineage avian influenza A (H7N9) virus were first reported in China. Annual epidemics of human infections with Asian H7N9 viruses in China driven mostly by exposure to infected poultry at live poultry markets have been reported since 2013. A small percentage of reported cases have been associated with possible limited, non-sustained human-to-human transmission, mostly occurring between family members. However, limited, non-sustained transmission of Asian H7N9 has been reported in a few cases in hospitals. (Xiang, N et al, 2016.”Assessing Change in Avian Influenza A(H7N9) Virus Infections During the Fourth Epidemic — China, September 2015–August 2016“; Chen et al. 2016. “Nosocomial Co-Transmission of Avian Influenza A(H7N9) and A(H1N1)pdm09 Viruses between 2 Patients with Hematologic Disorders;” Fang et al, 2015. “Nosocomial transmission of avian influenza A (H7N9) virus in China: epidemiological investigation.”

It is possible for human-to-human transmission of other non-human (animal-origin) influenza A viruses to range along a continuum; from occasional, limited, non-sustained human-to-human transmission of one or more generations without further spread (“dead-end transmission”), to efficient and sustained human-to-human transmission. Efficient and sustained (ongoing) transmission of non-human influenza A viruses (including avian influenza A viruses) among people in the community is needed for an influenza pandemic to begin.