Influenza viruses are identified by the combination of two proteins found on their surface; hemagglutinin and neuraminidase.
There are 16 known types of hemagglutinin (H) and 9 known types of neuraminidase (N). Theoretically, they can produce 144 different strains of influenza, although not all of these strains of influenza circulate in the wild or can infect humans.
In the relatively short history of our observation of the influenza virus, only the H1, H2, and H3 strains have adapted well to people.
On rare occasions humans have been infected by other influenza strains, such as H5N1, H7N7, and H9N2. These strains are primarily seen in birds, but since they are constantly evolving and mutating, the concern is one might eventually adapt to human hosts.
The OIE requires that all detected H5 and H7 infections in birds be reported, and that steps be made to control and eradicate outbreaks.
LPAI (Low Pathogenic Avian Influenza) – which normally does little harm to flocks or humans – has the potential to evolve to HPAI (Highly Pathogenic Avian Influenza) if not controlled, particularly when introduced into a crowded farming environment.
H9 viruses are not currently required to be reported, so we don’t have a very good handle on how prevalent it may be around the world.
Recent infections detected in two Hong Kong residents (see Another H9 Report From Hong Kong) who had visited mainland China showed that the H9N2 virus has mutated away from the existing vaccine candidate virus, which is based on a 1999 isolate.
If all of this sounds a bit ominous, it may help to know that all of the H9 human infections detected to date have been relatively mild, that human-to-human transmission has not been detected, and that the H9 virus has managed to circulate in birds for many years without sparking a pandemic.
Still, past performance is no guarantee of future results. The WHO recommends the creation of a new vaccine candidate based on this A/Hong Kong/33982/2009-like virus.
Antigenic and genetic characteristics of influenza A(H5N1) and influenza A(H9N2) viruses and candidate vaccine viruses developed for potential use in human vaccines
This summary provides a review on the influenza A(H5N1) and A(H9N2) virus activity and virus characterization, and describes the current status of the development of new A(H5N1) and A(H9N2) candidate vaccine viruses. It is meant to provide guidance for national authorities and vaccine companies on the selection of candidate viruses for use in vaccine development.