Researchers at OUCRU working with colleagues at the National Institute of Hygiene and Epidemiology have identified a cluster of seven cases of infection with the pandemic H1N1 influenza (swine flu) that are resistant to the drug oseltamivir – known also by its brand name Tamiflu.  Oseltamivir is an antiviral drug that reduces the severity and duration of influenza symptoms. Treatment usually lasts for five days and has the most effect when started within two days of the onset of symptoms. Globally, the number of oseltamivir resistant pandemic H1N1 viruses reported to date is small (less than 1% of viruses tested) and most cases have occurred whilst the patients were taking oseltamivir. Cases of resistance in pandemic H1N1 patients not on oseltamivir have been found before, including recent clusters in immunocompromised patients in special care units in Wales and North Carolina. However, this cluster in Vietnam occurred in healthy young adults and confirms that such resistant viruses can spread among healthy people who are not being treated. Thankfully, further cases related to this cluster have not been identified but vigilance is needed and measures should be taken to reduce the risk of resistant viruses emerging and spreading.