Sales of oseltamivir in Norway prior to the emergence of oseltamivir resistant influenza A(H1N1) viruses in 2007-08

Abstract

Background. An unprecedented high proportion of oseltamivir resistant influenza A(H1N1) viruses emerged in the 2007-08 influenza season. In Norway, two thirds of all tested A(H1N1) viruses were resistant to the antiviral drug. In order to see if this emergence could be explained by a drug induced selection pressure, we analysed data on the sales of oseltamivir in Norway for the years 2002-07. Methods. We used data from two sources; the Norwegian Drug Wholesales Statistics Database and the Norwegian Prescription Database (NorPD), for the years 2002-2007. We calculated courses sold of oseltamivir (Tamiflu ®) per 1000 inhabitants per year. Results. Our data showed that, except for the years 2005 and 2006, sales of oseltamivir were low in Norway; courses sold per 1000 inhabitants varied between 0.17-1.64. The higher sales in 2005 and 2006 we believe were caused by private stockpiling in fear of a pandemic, and do not represent actual usage. Conclusion. A drug induced selection pressure was probably not the cause of the emergence of oseltamivir resistant influenza A(H1N1) viruses in 2007-08 in Norway.