Antigenic and genetic characteristics of influenza A(H5N1) and influenza A(H9N2) viruses for development of candidate vaccines viruses for pandemic preparedness – February 2011 (March 11 2011)
[Source: World Health Organization, full PDF document (LINK
). Extract, edited.]
Weekly epidemiological record
Relevé épidémiologique hebdomadaire
11 march 2011, 86th year / 11 mars 2011, 86e année
No. 11, 2011, 86, 93–100
Antigenic and genetic characteristics of influenza A(H5N1) and influenza A(H9N2) viruses for development of candidate vaccines viruses for pandemic preparedness – February 2011
The development of representative influenza A(H5N1) and A(H9N2) candidate viruses for influenza vaccines, which is coordinated by WHO, remains an essential component of the overall global strategy for pandemic preparedness. Comparisons of the candidate vaccine viruses with respect to antigenicity and their relationship to newly emerging viruses continue, and information on these comparisons will be updated periodically by WHO. An update of current and completed clinical vaccine trials can be found on the WHO website.(1)
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 influenza 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 influenza A(H1N1) 2009 viruses, the zoonotic and pandemic threats posed by influenza A(H5N1) viruses remain. This summary provides an update on the characterization of influenza A(H5N1) viruses isolated from birds and humans, and the status of the development of candidate influenza A(H5N1) vaccine viruses.
Influenza A(H5N1) activity: 27 September 2010 to 15 February 2011
Influenza 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 – all of which have also reported infections in birds (Table 1).
Antigenic and genetic characteristics
A nomenclature for phylogenetic relationships among the haemagglutinin (HA) genes of influenza A(H5N1) viruses has been devised in consultation with representatives from the United Nations Food and Agriculture Organization (FAO), the World Organisation for Animal Health (OIE) and WHO. This nomenclature is updated when new genetic clades emerge, and can be found on the WHO website.(2)
Viruses characterized from 27 September 2010 to 15 February 2011 belonged to the following clades.
Influenza A(H5N1) candidate vaccine viruses
- Clade 1 viruses were detected in poultry and in a human case in Cambodia. Genetic characterization of these viruses showed that they were closely related to clade 1 viruses previously circulating in Cambodia (Figure 1).
- Clade 2.2.1 viruses continue to circulate in poultry in Egypt with sporadic transmission to humans. The HA genes of the Egyptian clade 2.2.1 viruses cluster in 2 major genetic groups, C and F (Figure 2). All recent human cases in Egypt occurring between September 2010 and February 2011 were caused by viruses belonging to group C, which in Egypt are most frequently found infecting backyard poultry. Viruses isolated during this period were genetically similar to those isolated previously. Recent human viruses reacted well to postinfection ferret antiserum raised against the candidate vaccine virus A/Egypt/N03072/2010 (Table 2).
- Clade 2.3.2 viruses were detected in wild birds in Hong Kong SAR, Japan and the Republic of Korea, and in poultry in Japan, Nepal, the Republic of Korea and Viet Nam. A case of human infection was also detected in Hong Kong SAR. Although the viruses isolated between September 2010 and February 2011are genetically similar to viruses isolated in 2009 and 2010, viruses within clade 2.3.2 form 2 distinct phylogenetic groups represented by A/Hubei/1/2010 and A/barn swallow/Hong Kong/D10-1161/2010 (Figure 3). The virus from the human case in 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 post-infection ferret antiserum raised to the candidate vaccine virus A/common magpie/Hong Kong/5052/2007 (Table 3).
- Clade 2.3.4 viruses were detected in poultry in Hong Kong SAR, Myanmar and Viet Nam. Although there is genetic heterogeneity amongst clade 2.3.4 viruses, as previously reported,3 the recent viruses are similar to those detected in previous years (Figure 1). The viruses detected in Hong Kong SAR and Viet Nam reacted well to post-infection ferret antisera raised against the candidate vaccine virus A/chicken/Hong Kong/AP156/2008, or against the candidate viruses A/duck/Laos/3295/2006 and A/Anhui/1/2005 (data not shown).
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 have been proposed. The available candidate influenza A(H5N1) vaccine viruses are listed in Table 4. On the basis of the geographical spread, epidemiology and antigenic and genetic properties of the influenza A(H5N1) viruses, national authorities may recommend the use of ≥1 of these candidate vaccine viruses to produce pilot lots of vaccines, clinical trials and stockpiling of influenza A(H5N1) vaccines.
For pandemic preparedness purposes, new influenza A(H5N1) candidate vaccine viruses will be developed as the viruses continue to evolve; these will be 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 website.(4)
Influenza A(H9N2) viruses are enzootic in poultry populations in parts of Asia and the Middle East. Although characterization data on recent influenza A(H9N2) viruses from many regions are limited, the majority of viruses that have been sequenced belong to the G1 or chicken/Beijing (Y280/G9) clades. Influenza A(H9N2) viruses analysed from these clades react well to postinfection ferret antisera raised to the candidate vaccine viruses of the corresponding clades (Table 5). Since 1999, when the first human infection was detected, isolation of influenza A(H9N2) viruses from humans and swine has been reported infrequently. In all human cases, the associated disease symptoms have been mild and there has been no evidence of human-to-human transmission.
Human influenza A(H9N2) infection: 27 September 2010 to 15 February 2011
No human cases of infection with influenza A(H9N2) have been detected during this reporting period. Work continues on the A/Hong Kong/33982/2009 candidate vaccine virus proposed during the WHO consultation on the composition of influenza vaccines for the northern hemisphere for 2010–2011.(5) (Table 5).
(1) Tables on the clinical trials of pandemic influenza prototype vaccines. Geneva, World Health Organization, 2010 (LINK
), accessed February 2011).
(2) Continuing progress towards a unified nomenclature system for the highly pathogenic H5N1 avian influenza viruses. Geneva, World Health Organization, 2009 (LINK
), accessed February 2011).
(3) See No. 42, 2010, pp. 418–424.
(4) Avian influenza: guidelines, recommendations, descriptions. Geneva, World Health Organization, 2011 ((LINK
), accessed February 2011).
(5) See No. 11, 2010, pp. 100–107.