13 June 2008

The detection of an increased number of A(H1N1) viruses with resistance to oseltamivir was initially reported to WHO by Norway on 25 January 2008. The viruses carried a specific neuraminidase (NA) mutation (H274Y) that confers high-level resistance to oseltamivir in N1-containing influenza viruses¹ . Prior to the recent report from Norway, such resistance was rarely observed in community isolates of influenza A or B. During the previous northern hemisphere winter season (2006/2007), surveillance through the Global Influenza Surveillance Network (GISN) laboratories found no oseltamivir-resistant H1N1 viruses among isolates from Japan or Europe, and less than 1% prevalence among H1N1 isolates from the United States of America.