Vaccine Resistant Asymptomatic H5N1 in China Raises Concerns
Recombinomics Commentary 15:47
January 19, 2009
The results showed that the virus did not happen with the human influenza virus re-allocation, which proved the source of infection is still Huang poultry source.
The above comments attributed to the Chinese CDC indicate the sequences from the fatal case in Beijing (A/Beijing/1/2009) do not have evidence of reassortment, but there are small variations. The relationship of these sequences to prior human isolates in China remains unclear.
Most cases in China are clade 2.3.4 (Fujian strain), which was isolated in Fujian Province 2005 from poultry, but links back to H5N1 from Fujian province in 2003. Similarly clade 7 has been isolated between 2005 and 2008 in northern China and Hunan, but links back to a fatal case in Beijing in 2003.
However, the most recent reported H5N1 outbreaks in poultry in China were in Jiangsu in December, 2008. Although more than 300,000 birds were culled, there were no reported deaths listed in the OIE report, indicating the poultry infection was asymptomatic and sequence analysis indicated the H5N1 was clade 7.
That outbreak has been followed by found confirmed cases in China (see updated map) in the past two weeks in Beijing, Shandong, Guizhou, and Hunan. Although the human cases were widespread throughout China, there have been no confirmations of H5N1 in poultry in these areas, raising concerns that H5N1 was being transmitted to these victims from asymptomatic poultry.
Asymptomatic poultry can be generated by mismatched vaccines. The cross reactivity of the vaccination keeps the host healthy, but virus is shed, raising concerns of spread to unvaccinated hosts, such as wild birds or humans.
Moreover, co-circulation of H5N1 can give rise to evolved versions of H5N1 that can have altered tissue tropism or host ranges, especially in changes in the receptor binding domain are acquired.
Clade 7 sequences from the 2006 outbreak in Shanxi in 2006 have been released, and these sequences are distinct from clade 1 or clade 2, and have a number of receptor binding domain changes. These changes could switch genetic backgrounds via recombination, as has been seen in anti-vital resistance in seasonal flu, where receptor binding domain changes in H3N2 and H1N1 have been associated worldwide fixing of H274Y in NA or S31N in M2.
Release of sequences from the poultry outbreaks in Jiangsu, as well as confirmed human cases this month, would be useful.
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