not available CS9.4 Antiviral treatment of H1N1: Virological response and resistance P.K.S. Chan1 *, N. Lee2, D. Hui2. 1Department of Microbiology, and 2Medicine and Therapeutics, The Chinese University of Hong Kong, Hong Kong S.A.R. Influenza causes annual recurrent health impact on the society. In Hong Kong, about 130 580 per 100,000 elderly individuals are hospitalized annually for influenzaassociated complications such as pneumonia, exacerbation of chronic obstructive pulmonary diseases or asthma, and acute cardiovascular events. While antiviral agents specific for influenza have been available for many years, the practice of antiviral treatment for these hospitalized patients still varies from centre to centre. On one extreme, the duration of 48 hours after illness onset is still regarded as the “cut-off” for cost-effective treatment. Whereas, in view of the often higher viral load and prolonged shedding of viruses seen in patients with severe disease, some clinicians would offer treatment even beyond the golden period of “48 hours”. We have shown that treatment with neuraminidase inhibitor for the high-risk group hospitalized for laboratory confirmed seasonal influenza could accelerate viral clearance, shorten viral shedding, reduce the length of hospital stay, allow earlier discontinuation of oxygen therapy and decrease mortality. For instance, we have shown that initiation of oseltamivir within 96 hours after illness onset was associated independently with a decrease in mortality (OR: 0.26, 95%CI: 0.08 0.87). Our experience from the 2009 pandemic suggested that patients with severe pneumonia often had a higher viral load at presentation (1.3 logs higher than those with mild infection), and most of them had viral RNA detected from extrapulmonary sites. Clinical recovery was associated with rapid viral clearance following a standard course of neuraminidase inhibitor treatment. However, patients with severe pneumonia often exhibited a prolonged course of viral shedding, especially from the lower respiratory tract, despite the administration of neuraminidase inhibitor. This raised the consideration of higher dose and more sustained antiviral regime for selected high-risk patients. The development of drug resistance for a highly mutable virus like influenza is always a concern. The older class of anti-influenza agent, adamantanes, is well known for such problem. Given the known resistance of the 2009 pandemic H1N1 virus to this class of anti-influenza compounds, the choice of treatment and prophylaxis is limited to neuraminidase inhibitors. While a huge number of doses of neuraminidase inhibitors, mainly oseltamivir and to a small extent zanamivir, have been administered during the pandemic; resistance remains at a low level as what one would expect from the natural mutation of this virus. Majority of the reported cases of neuraminidase inhibitorresistance were from low-dose or prolong administration in immunosuppressed patients. The administration of effective antiviral compounds remains an important component in the clinical management of influenza infection, especially for those with a high risk of developing severe complications. State-of-the Art Lecture 4 Saturday, July 15, 2011, 11:15 11:45