Global Review of recent Antiviral Resistance data
World Health Organization (WHO), Geneva, February 2011
Each year in February WHO convenes a meeting with advisers from the WHO Influenza Collaborating Centres and Essential Reference laboratories in order to achieve consensus and advise WHO on the optimal formulation of seasonal influenza vaccines for the next influenza season. The meeting held in Geneva in February 2011 noted the diversity of experience this season so far with North America and parts of North Asia experiencing outbreaks of influenza A(H3N2) and some A(H1N1)2009 and B viruses while Europe experiencing mainly influenza A(H1N1)2009 and B viruses; the former seasonal influenza A(H1N1) has been detected only in few samples globally.
The meeting also reviewed the recent global observations on antiviral resistance. There was very little resistance to oseltamivir and none to zanamivir in 2010-2011 in A(H1N1)2009 viruses, and most were linked to the use of oseltamivir for prophylaxis or treatment. However, in some countries e.g. Japan and the United Kingdom, there were a few cases with no known exposure to oseltamivir. There were no reports of oseltamivir-resistant A(H3N2) or B viruses and no zanamivir-resistant viruses were detected. Updates are available on the WHO website, the latest of which is accessible here.
Concerning M2 inhibitors, all A(H1N1)2009 and A(H3N2) viruses tested were resistant to the M2 inhibitors, amantadine and rimantadine.
The full report summarizing the data evaluated in February 2011 is available at: http://www.who.int/csr/disease/influenza/2011_02_recommendation.pdf
In addition, the lists of vaccine candidate viruses are available at: http://www.who.int/csr/disease/influenza/vaccinerecommendations2/en/index.html
ECDC comment (3rd March 2011):
As mentioned above, there was very little resistance to oseltamivir in A(H1N1)2009 viruses, and most were linked to the use of oseltamivir for prophylaxis or treatment. However, a recent paper from the UK published in the journal Euro Surveillance  claims that, in this country and during the winter period 2010/11, twenty-seven epidemiologically unlinked, confirmed cases of oseltamivir-resistant influenza A(H1N1)2009 virus infection were detected in multiple, geographically dispersed settings. The interesting finding is that three of these cases were found in community settings, with no known exposure to oseltamivir.
ECDC is monitoring the influenza virus antiviral resistance in the EU/EEA countries and publishes an update weekly in the WISO. The antiviral resistance situation has been commented also in the monthly virology report of March 2011. The results showed on this report only reflect the analysis of a subset of samples centrally tested.
 - Lackenby A, Moran Gilad J, Pebody R, Miah S, Calatayud L, Bolotin S, Vipond I, Muir P, Guiver M, McMenamin J, Reynolds A, Moore C, Gunson R, Thompson C, Galiano M, Bermingham A, Ellis J, Zambon M. Continued emergence and changing epidemiology of oseltamivir-resistant influenza A(H1N1)2009 virus, United Kingdom, winter 2010/11. Euro Surveill. 2011;16(5):pii=19784. Available online: http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19784