Tamiflu (Oseltamivir) has been an effective treatment for the new H1N1 flu virus. This may not continue to be true. Recently, Tamiflu-resistant strains of the new H1N1 have been identified in people from Japan, Denmark and the United States (US News and World Report).
Two people from Australia were treated with Tamiflu for 7 days in Vietnam, but continued to shed virus. The sequence of these isolates have not yet been released, but it seems probable that they will turn out to be Tamiflu-resistant (Vietnamnet).
Tamiflu-resistant strains of both seasonal influenza and H5N1 (bird flu) have previously been identifed. Until recently, it was thought that these variants were not evolutionarily fit and thus could not spread efficiently. However, in the past year, Tamiflu-resistant seasonal H1N1 virus comprised nearly 100% of the variants observed in the US and many areas of the world. The reason for this unprecedented spread of Tamiflu-resistant seasonal flu is not known.
Estimates vary on the case fatality rate for the new H1N1 virus. However, there is good evidence to suggest that the use of Tamiflu is preventing more severe symptoms from being observed in patients who receive it within the first 48 hours. Most of the deaths have been observed in people who did not receive such treatment.
We do not know if Tamiflu-resistant new H1N1 will spread as effectively as Tamiflu-resistant seasonal H1N1. But if it does, it is likely that the percent of people infected who die will increase. Perhaps by a lot.