RESUMO
BACKGROUND: The first case of influenza A(H1N1)pdm09 was detected in Pakistan in June 2009. Since then, it has continued to circulate causing considerable morbidity and mortality. The purpose of this study was to evaluate the evolutionary changes in influenza A(H1N1)pdm09 viruses from 2009 to 2016 and their relevance to the current vaccine viruses. METHODS: Respiratory specimens (throat or nasopharyngeal swabs) were collected from patients with influenza-like illness and severe acute respiratory illness. Samples were processed following the protocol of the US Centers for Disease Control and Prevention. Sequencing and phylogenetic analysis of Haemagglutinin and neuraminidase genes were carried out on representative isolates of Pakistan viruses. RESULTS: Between January 2009 and February 2016, out of 16,024 samples analysed, 1950 (12%) were positive for influenza A. During the pandemic period (2009-2010), influenza A(H1N1)pdm09 was the dominant strain with 366 out of 808 (45%) total influenza positive cases. In the post-pandemic period (2011-2016), a total of 1078 out of 1911 (56%) cases were positive for influenza A(H1N1)pdm09 with co-circulation of different influenza A subtypes. The Pakistan A(H1N1)pdm09 viruses belonged to two genetic clades: clade 7 in the pandemic period, and clade 7 (2011) and clade 6B (2015) in the post-pandemic period. Sequence analysis of genes coding for surface glycoprotein's of Haemagglutinin and neuraminidase had a high degree of sequence similarity with corresponding genes of regional viruses circulating in South-East Asia. CONCLUSION: Influenza A(H1N1)pdm09 viruses from Pakistan clustered into two genetic clades, with co-circulation of some variants. Key substitutions in the receptor binding site and a few changes indicative of virulence were also detected in the post-pandemic strains. Continued monitoring of the viruses is essential for early identification of potential variants of high virulence and their relevance to current vaccine strains.
