title: Influenza A(H1N1) Oseltamivir Resistant Viruses in the Netherlands During the Winter 2007/2008.
abstract: BACKGROUND
  Antiviral susceptibility surveillance in the Netherlands was intensified after the first reports about the emergence of influenza A(H1N1) oseltamivir resistant viruses in Norway in January, 2008.
  
  
  METHODS
  Within the existing influenza surveillance an additional questionnaire study was performed to retrospectively assess possible risk factors and establish clinical outcome of all patients with influenza virus A(H1N1) positive specimens. To discriminate resistant and sensitive viruses, fifty percent inhibitory concentrations for the neuramidase inhibitors oseltamivir and zanamivir were determined in a neuraminidase inhibition assay. Mutations previously associated with resistance to neuramidase inhibitors and M2 blockers (amantadine and rimantadine) were searched for by nucleotide sequencing of neuraminidase and M2 genes respectively.
  
  
  RESULTS
  Among 171 patients infected with A(H1N1) viruses an overall prevalence of oseltamivir resistance of 27% (95% CI: 20-34%) was found. None of influenza A(H1N1) oseltamivir resistant viruses tested was resistant against amantadine or zanamivir. Patient characteristics, underlying conditions, influenza vaccination, symptoms, complications, and exposure to oseltamivir and other antivirals did not differ significantly between patients infected with resistant and sensitive A(H1N1) viruses.
  
  
  CONCLUSIONS
  In 2007/2008 a large proportion of influenza A(H1N1) viruses resistant to oseltamivir was detected. There were no clinical differences between patients infected with resistant and sensitive A(H1N1) viruses. Continuous monitoring of the antiviral drug sensitivity profile of influenza viruses is justified, preferably using the existing sentinel surveillance, however, complemented with data from the more severe end of the clinical spectrum. In order to act timely on emergencies of public health importance we suggest setting up a surveillance system that can guarantee rapid access to the latter.
authors.affiliation: Centre for Infectious Disease Control, National Institute for Public Health and the Environment (RIVM), Bilthoven, The Netherlands.
authors.forename: Frederika
authors.initials: F
authors.lastname: Dijkstra
authors.forename: Marcel
authors.initials: M
authors.lastname: Jonges
authors.forename: Ruud
authors.initials: R
authors.lastname: van Beek
authors.forename: Gé A
authors.initials: GA
authors.lastname: Donker
authors.forename: François G
authors.initials: FG
authors.lastname: Schellevis
authors.forename: Marion
authors.initials: M
authors.lastname: Koopmans
authors.forename: Marianne A B
authors.initials: MA
authors.lastname: van der Sande
authors.forename: Albert D M E
authors.initials: AD
authors.lastname: Osterhaus
authors.forename: Charles A B
authors.initials: CA
authors.lastname: Boucher
authors.forename: Guus F
authors.initials: GF
authors.lastname: Rimmelzwaan
authors.forename: Adam
authors.initials: A
authors.lastname: Meijer