Does the virus spread easily from birds to humans?
No. Though more than 100 human cases have occurred in the
current outbreak, this is a small number compared with the huge
number of birds affected and the numerous associated
opportunities for human exposure, especially in areas where
backyard flocks are common. It is not presently understood why
some people, and not others, become infected following similar
exposures.

What about the pandemic risk?
A pandemic can start when three conditions have been met: a new
influenza virus subtype emerges; it infects humans, causing serious
illness; and it spreads easily and sustainably among humans. The
H5N1 virus amply meets the first two conditions: it is a new virus for
humans (H5N1 viruses have never circulated widely among
people), and it has infected more than 100 humans, killing over half
of them. No one will have immunity should an H5N1-like pandemic
virus emerge.

All prerequisites for the start of a pandemic have therefore been met
save one: the establishment of efficient and sustained human-to-
human transmission of the virus. The risk that the H5N1 virus will
acquire this ability will persist as long as opportunities for human
infections occur. These opportunities, in turn, will persist as long as
the virus continues to circulate in birds, and this situation could
endure for some years to come.


What changes are needed for H5N1 to become a pandemic virus?
The virus can improve its transmissibility among humans via two
principal mechanisms. The first is a “reassortment” event, in which
genetic material is exchanged between human and avian viruses
during co-infection of a human or pig. Reassortment could result in
a fully transmissible pandemic virus, announced by a sudden surge
of cases with explosive spread.

The second mechanism is a more gradual process of adaptive
mutation, whereby the capability of the virus to bind to human cells
increases during subsequent infections of humans. Adaptive
mutation, expressed initially as small clusters of human cases with
some evidence of human-to-human transmission, would probably
give the world some time to take defensive action.

What is the significance of limited human-to-human
transmission?
Though rare, instances of limited human-to-human transmission of
H5N1 and other avian influenza viruses have occurred in
association with outbreaks in poultry and should not be a cause for
alarm. In no instance has the virus spread beyond a first generation
of close contacts or caused illness in the general community. Data
from these incidents suggest that transmission requires very close
contact with an ill person. Such incidents must be thoroughly
investigated but – provided the investigation indicates that
transmission from person to person is very limited – such incidents
will not change the WHO overall assessment of the pandemic risk.

There have been a number of instances of avian influenza infection
occurring among close family members. It is often impossible to
determine if human-to-human transmission has occurred since the
family members are exposed to the same animal and
environmental sources as well as to one another.
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