Molecular characterization of influenza A(H1N1)pdm09 virus circulating during the 2009 outbreak in Thua Thien Hue, Vietnam.

INTRODUCTION The influenza A(H1N1)pdm09 virus arrived in Vietnam in May 2009 via the United States and rapidly spread throughout the country. This study provides data on the viral diagnosis and molecular epidemiology of influenza A(H1N1)pdm09 virus isolated in Thua Thien Hue Province, central Vietnam. METHODOLOGY Nasopharyngeal swabs and throat swabs from 53 clinically infected patients in the peak of the outbreak were processed for viral diagnosis by culture and RT-PCR. Sequencing of entire HA and NA genes of representative isolates and molecular epidemiological analysis were performed. RESULTS A total of 32 patients were positive for influenza A virus by virus culture and/or RT-PCR; of these 22 were positive both by viral isolation and RT-PCR, 2 only by virus culture and 8 only by RT-PCR. The novel subtype of influenza A(H1N1)pdm09 was present in 93.4% of the isolates. Phylogenetic analysis of the HA and NA gene sequences showed identities higher than 99.50% in both genes. They were also similar to reference isolates in HA sequences (>99% identity) and in NA sequences (>98.50% identity). Amino acid sequences predicted for the HA gene were highly identical to reference strains. The NA amino acid substitutions identified did not include the oseltamivir-resistant H275Y substitution. CONCLUSION viral isolation and RT-PCR together were useful for diagnosis of the influenza A(H1N1)pdm09 virus. Variations in HA and NA sequences are similar to those identified in worldwide reference isolates and no drug resistance was found.