A Community Cluster of Oseltamivir-Resistant Cases of 2009 H1N1 Influenza

Abstract

This report summarizes a cluster of 7 cases of oseltamivir resistant 2009 H1N1 infection in Vietnam. In July 2009, 10 students socialized in the same train carriage. Fever developed in 4 of the students within 12 hours and in another 2 within 48 hours after arrival. An additional case was identified in a traveller from a different carriage. Nasal and/or throat swabs from the patients were positive for 2009 H1N1 RNA and viruses were cultured from specimens of 3 persons. The H275Y substitution was detected retrospectively in all subjects before oseltamivir treatment. The concentrations of oseltamivir carboxylate required for a 50% inhibition of neuraminidase activity of the isolated viruses were 323.6, 429.5, and 889.2 nM; these concentrations confirmed resistance. All patients were subsequently treated with oseltamivir phosphate (75 mg twice daily) and eventually recovered. However, the patient with the highest 50% inhibitory concentration continued to test positive on RT-PCR until day 9. This case shows that resistant 2009 H1N1 viruses are transmissible and can replicate and cause illness in healthy people in the absence of selective drug pressure.