Background on Human Infections with other Avian Influenza Viruses
Human infections with avian influenza viruses are rare and most often occur after people are in contact with an infected bird. However, non-sustained person-to-person spread of other avian influenza viruses is thought to have occurred in the past, most notably with avian influenza A (H5N1) viruses.
- In 1997, when the H5N1 virus first emerged there was evidence of limited transmission of H5N1 virus to health care workers and household contacts of H5N1 patients in Hong Kong, but the virus did not spread further.
- In 2003, in the Netherlands, there was evidence of possible transmission of H7N7 from 2 poultry workers to 3 family members. All 3 family members had conjunctivitis and one also had ILI. (M Du Ry van Beest Holle, Meijer, et al, 2005. “Human to Human Transmission of of Avian Influenza /H7N7, The Netherlands, 2004.”)
- In 2004, in Thailand, there was evidence of probable human-to-human spread in a family cluster. Transmission was associated with prolonged very close contact between an ill child and her mother and her aunt. Transmission did not spread beyond one person. (Ungchusak et al, 2005. “Probable Person-to-Person Transmission of Avian Influenza A (H5N1).”)
- In 2005, in Indonesia, limited person-to-person H5N1 transmission could not be excluded in two clusters among 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 December 2007, limited, unsustained spread is thought to have occurred between a sick son and his father. (Wang et al, 2008. “Probable limited person-to-person transmission of highly pathogenic avian influenza A (H5N1) virus in China.”)
- Also in 2007, evidence gathered during an outbreak investigation in Pakistan supports the theory of initial transmission from poultry to humans followed by human-to-human transmission involving a third generation. (WHO, Weekly Epidemiological Record, 2008. “Human cases of avian influenza A(H5N1) in North-West Frontier Province, Pakistan, October–November 2007
Almost all of these cases occurred during unprotected, close and prolonged contact between a caregiver (mostly blood-related family members) and a very ill patient.
Based on this previous experience, it’s likely that some limited human-to-human spread of this H7N9 virus will be detected.
Human-to-human transmission ranges along a continuum; from occasional, “dead-end” human-to-human transmission, to efficient and sustained human-to-human transmission. “Dead end” transmission usually refers to when a virus from an animal host infects a person and then there is some subsequent transmission that eventually burns out. Efficient and sustained (ongoing) transmission in the community is needed for an influenza pandemic to begin. There is no evidence that the H7N9 virus in China is spreading in a sustained, ongoing way at this time.
However, the concern is that this H7N9 virus might either adapt to allow efficient transmission during the infection of mammals or reassort its gene segments with human influenza viruses during the co-infection of a single host, resulting in a new virus that would be transmissible from person to person. Such events are believed to have preceded the influenza pandemics of 1918, 1957, and 1968.