Fresh doubts were cast on the efficacy of Tamiflu as a treatment for bird flu on Wednesday night when one of the world’s most prestigious medical journals published new reports of resistance to the drug and deaths in patients in Vietnam.

Menno de Jong and colleagues from the hospital for tropical diseases in Ho Chi Minh City recorded in the New England Journal of Medicine that four out of eight patients suffering from the H5N1 flu strain and treated with Tamiflu had died, including two who developed resistance.