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:

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.