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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.

For example:

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.