Influenza A (H5N1) virus with an amino acid substitution in neuraminidase conferring high-level resistance to oseltamivir was isolated from two of eight Vietnamese patients during oseltamivir treatment. Both patients died of influenza A (H5N1) virus infection, despite early initiation of treatment in one patient. Surviving patients had rapid declines in the viral load to undetectable levels during treatment. These observations suggest that resistance can emerge during the currently recommended regimen of oseltamivir therapy and may be associated with clinical deterioration and that the strategy for the treatment of influenza A (H5N1) virus infection should include additional antiviral agents.
Addendum: Today's Guardian presents the UK government's reaction to the study. Here's an excerpt:
"Whilst there is some anecdotal evidence of the build-up of resistance to antiviral drugs such as Tamiflu, at present the experience is that these drugs do work and that they should work against a pandemic strain," a Department of Health spokeswoman said.