WHO: Antigenic and genetic characteristics of influenza A(H5N1) and influenza A(H9N2) viruses for the development of candidate vaccine viruses for pan

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Influenza A(H5N1)
Since 2003, highly pathogenic avian influenza A(H5N1) viruses have become enzootic in some countries and continue to cause outbreaks in poultry as well as sporadic human infections. The A(H5N1) viruses have diversified both genetically and antigenically leading to the need for multiple candidate vaccine viruses for pandemic preparedness purposes.

Despite the emergence of the pandemic 2009 influenza A(H1N1) [A(H1N1)pdm09]2 viruses, the zoonotic and pandemic threats posed by A(H5N1) viruses remain. This summary provides an update on the characterization of A(H5N1) viruses isolated from birds and humans, and the current status of the development of candidate A(H5N1) vaccine viruses.
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Influenza A(H5N1) activity from 27 September 2010 to 15 February 2011
A(H5N1) viruses have been detected in birds in Africa and Asia. Human infections have been reported to WHO from Cambodia, China Hong Kong Special Administrative Region (Hong Kong SAR), Egypt and Indonesia, countries, areas or territories that have also reported infections in birds (Table 1).

Antigenic and genetic characteristics
A nomenclature for phylogenetic relationships among the haemagglutinin (HA) genes of A(H5N1) viruses was devised in consultation with representatives of the Food and
Agriculture Organization of the United Nations (FAO), the World Organisation for Animal Health (OIE) and WHO. This nomenclature is updated when new genetic clades emerge and can be found on WHO website3.
Viruses characterized from 27 September 2010 to 15 February 2011 belonged to the
following clades.
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Clade 2.3.2 viruses were detected in wild birds in Hong Kong SAR, Japan and Republic of Korea, and in poultry in Japan, Nepal, Republic of Korea and Viet Nam. A human case was also detected in Hong Kong SAR. Although the recent viruses are genetically similar to viruses isolated in 2009 and 2010, viruses within clade 2.3.2 form two distinct phylogenetic groups represented by A/Hubei/1/2010 and A/barn swallow/Hong Kong/D10-1161/2010
(Figure 3). The human case from Hong Kong SAR in 2010 belonged to the A/Hubei/1/2010 group. Antigenically, the clade 2.3.2 viruses are heterogeneous and not all viruses react well to postinfection ferret antiserum raised to the candidate vaccine virus A/common
magpie/Hong Kong/5052/2007 (Table 3).
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Influenza A(H5N1) candidate vaccine viruses
Based on the current antigenic, genetic and epidemiological data, further clade 2.3.2 candidate vaccine viruses are recommended. Candidate vaccine viruses based on an A/Hubei/1/2010-like virus and an A/barn swallow/Hong Kong/D10-1161/2010-like virus are proposed. The available candidate A(H5N1) vaccine viruses are listed in Table 4. On the basis of the geographical spread, epidemiology and antigenic and genetic properties of the A(H5N1) viruses, national authorities may recommend the use of one or more of these candidate vaccine viruses for pilot lot vaccine production, clinical trials and stockpiling of A(H5N1) vaccines.

For pandemic preparedness purposes, new A(H5N1) candidate vaccine viruses will be
developed as the viruses continue to evolve and announced as they become available.
Institutions, companies and others who wish to receive these candidate vaccine viruses
should contact the WHO Global Influenza Programme at GISN@who.int or the institutions
listed in announcements published on the WHO website4.
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