What changes are needed for H5N1 or another avian influenza virus to cause a pandemic?

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Answer:

Three conditions must be met for a pandemic to start:

1. a new influenza virus subtype must emerge for which there is little or no human immunity;
2. it must infect humans and cause illness; and
3. it must spread easily and sustainably (continue without interruption) among humans.

The H5N1 virus in Asia, Europe, and Africa meets the first two conditions: it is a new virus for humans (H5N1 viruses have never circulated widely among people), and it has infected a growing number of people, killing over half of those known to have been infected. However, the third condition, the establishment of efficient and sustained human-to-human transmission of the virus, has not occurred. For this to take place, the H5N1 virus would need to improve its transmissibility among humans.

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Last Updated: 04/03/2007