Substandard poultry vaccination programs may cause higher mutations rates in the bird flu virus, rendering the vaccine ineffective and increasing the threat of cross-species transmission.
The H5N1 Type A influenza is mutating faster in countries that have been implementing wide-scale, but incomplete, vaccinations of poultry, according to results of a study published by Science Direct.
Highly pathogenic avian influenza (HPAI) H5N1 was introduced into Egypt in early 2006. Despite the control measures taken, including mass vaccination of poultry, the virus rapidly spread among commercial and backyard flocks. Since the initial outbreaks, the virus in Egypt has evolved into a third order.
To better understand the dynamics of HPAI H5N1 evolution in countries that differ in vaccination policy, the researchers undertook an in-depth analysis of those virus strains circulating in Egypt between 2006 and 2010, and compared countries where vaccination was adopted (Egypt and Indonesia) to those where it was not (Nigeria, Turkey and Thailand).
The analysis revealed that two main Egyptian subclades (termed A and B) have co-circulated in domestic poultry since late 2007 and exhibit different profiles of positively selected codons and rates of nucleotide substitution. Although the direct association between H5N1 vaccination and virus evolution is difficult to establish, evidence was found for a difference in the evolutionary dynamics of H5N1 viruses among countries where vaccination was or was not adopted.
In particular, both evolutionary rates and the number of positively selected sites were higher in virus populations circulating in countries applying avian influenza vaccination for H5N1, compared to viruses circulating in countries which had never used vaccination.
The study concluded therefore that a greater consideration is needed of the potential consequences of inadequate vaccination on viral evolution.
Source: The Scientist