Peer-reviewed literature

06 January 2012

Report of a large cluster of oseltamivir resistant viruses in Australia

On December 29, 2011, Australian investigators released a summary account of the characterization of an outbreak with sustained community transmission of oseltamivir-resistant influenza A(H1N1)pdm09 viruses in the state of New South Wales. The report focuses on 29 oseltamivir-resistant cases identified from 182 patients in the Hunter New England region of the state between May and August 2011. Only one of these 29 patients had received oseltamivir before specimen collection. Hemagglutinin and neuraminidase analysis showed that resistant strains were closely related, which indicates the spread of a single A(H1N1)pdm09 variant. An epidemiological link was demonstrated between 10 of the 29 cases. The report describes the significantly higher IC50 values of oseltamivir-resistant viruses to oseltamivir and peramivir compared to wild-type strains but also indicates that oseltamivir-resistant viruses were as susceptible to zanamivir as wild-type strains. The report also states that the resistant strains are antigenically similar to the vaccine strain including in patients who received the vaccine.


While this outbreak of oseltamivir-resistant influenza viruses raised great concern when first reported, it is notable that since August 2011 no new cases associated with this outbreak have been detected. Globally, the proportion of viruses with oseltamivir resistance remains about 1% in untreated patients and transmission has been reported only in close settings. Much higher proportions of resistant viruses are detected in some situations such as prolonged treatment in individuals with immunodeficiency. For the moment, clinicians should continue to use oseltamivir for treatment of high-risk and severe cases of influenza. However, this event illustrates the need for continued vigilance so that treatment guidelines can be amended when necessary. WHO continues to recommend that countries who have the means to test for oseltamivir-resistant influenza A(H1N1)pdm09 viruses do so in a sample of cases and that all resistant viruses should be sent to a WHO collaborating center for further characterization. In addition, cases with resistant virus should be investigated for evidence of onward transmission and any findings to suggest that transmission of resistant virus is occurring should be immediately reported to WHO.


Hurt, A.C., et. al. Community Transmission of Oseltamivir-Resistant A(H1N1)pdm09 Influenza. NEJM, 2011:2541-2542;365(26).

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