Ethical clearance for the study was obtained from, 'Institutional Human Ethical Committee' as part of the pandemic influenza investigations. Written consent was obtained from all the participants involved in the study. For minors and critically ill patients it was obtained from parent/guardian. Patients confirmed to have p-H1N1-09 infection by a positive real time PCR test (http://www.who.int/csr/resources/publications/swineflu/CDC RealtimeRTPCRprotocol_SwineH1Ass-2009_20090428) were studied. These included 15 patients admitted to Intensive Care Unit and requiring mechanical ventilator support and 26 suffering from mild respiratory symptoms. All the mild cases were ambulatory patients admitted to a designated hospital. In the initial phases of the pandemic during which this study was performed, patients suggestive of Influenza-like illness were admitted to designated ward of a Corporation hospital. Throat swabs were collected and sent to the National Institute of Virology for diagnosis. Osletamivir treatment was initiated immediately after the confirmation of diagnosis. These patients were discharged after the completion of the treatment. On the contrary, majority of the severe patients were admitted only after the development of serious respiratory consequences. Same protocol was followed for diagnosis and antiviral treatment.