Oseltamivir-resistant influenza A(H1N1) pdm09 virus: first reported case from India.

Background: Recent studies on antiviral susceptibiliy from South-East Asia, Europe and the United States have shown sporadic neuraminidase inhibitor (NAI) resistance in A(H1N1)pdm09 viruses. We undertook a study to evaluate NAI resistance in these viruses isolated in India. Methods: Pandemic influenza viruses, isolated from 2009 to 2013, along with clincal samples were genetically analysed for known resistance markers in the neuraminidase (NA) gene. Clinical samples (n=1524) were tested for H275Y (N1 numbering; H274Y in N2 numbering) mutation by real time reverse transcriptase PCR (rRT-PCR). One hundred and ten randomly selected resistant and sensitive viruses were analysed by phenotypic assay. Results: All but one of the 2013 A(H1N1)pdm09 isolates were sensitive to oseltamivir. Genetic analysis of this isolate as well as the original clinical material showed that the presence of H275Y mutation was responsible for reduced susceptibility to oseltamivir in the patient. This was confirmed by phenotypic assay. Conclusion: The emergence of a pandemic influenza strain resistant to oseltamivir emphasizes the need for monitoring antiviral resistance as part of the National Influenza Programme in India.