Presence of oseltamivir-resistant pandemic A/H1N1 minor variants before drug therapy with subsequent selection and transmission.

A small proportion (1%-1.5%) of 2009 pandemic influenza A/H1N1 virus strains (A[H1N1]pdm09) are oseltamivir resistant, almost exclusively because of a H275Y mutation in the neuraminidase protein. However, many individuals infected with resistant strains had not received antivirals. Whether drug-resistant viruses are initially present as minor variants in untreated individuals before they emerge as the dominant strain in a virus population is of great importance for predicting the speed at which resistance will arise. To address this issue, we used ultra-deep sequencing of viral populations from serial nasopharyngeal specimens from an immunocompromised child and from 2 individuals in a household outbreak. We observed that the Y275 mutation was present as a minor variant in infected hosts before the onset of therapy. We also found evidence for the transmission of this drug-resistant variant with drug-susceptible viruses. These observations provide important information on the relative fitness of the Y275 mutation in the absence of oseltamivir treatment.