RESUMO
The H1N1 2009 influenza pandemic took the health care workers by surprise in spite of warning about influenza pandemic. Influenza A virus has the ability to overcome immunity from previous infections through the acquisition of genetic changes by shift or drift. Thus, understanding the evolution of the viruses in human is important for the surveillance and the selection of vaccine strains. A total of 23 pandemic A/H1N1 2009 viral HA gene sequences were downloaded from NCBI submitted during March and May 2010 by NIV and were analysed. Along with that the vaccine strain A/California/07/2009 was also downloaded from NCBI. All the sequences were used to analyse the evolution of the haemagglutinin (HA) by phylogenetic analysis. The HA gene could be divided into four groups with shift from 1 to lV revealing that the HA genes of the influenza A viruses evolved in a sequential way, in comparison to vaccine strain A/California/07/2009. Amino acid sequence analysis of the HA genes of the A/H1N1 2009 isolates, revealed mutations at positions 100, 220 and additional mutations in different positions 114, 171, 179, 190, 208, 219, 222, 239, 240, 247, 251, 260 and 285 .The mutations identified showed the adaptation of the new virus to the host that could lead to genetic changes inherent to the virus resulting in a reassortant which could be catastrophic, hence continuous monitoring of strains is mandatory.