Oseltamivir-Resistant H1N1 Identified in Australia

Between May and August 2011, resistant viruses with H275Y substitution ID'd in 29 of 182 patients

THURSDAY, Dec. 29 (HealthDay News) -- Oseltamivir-resistant prepandemic seasonal influenza 1 (H1N1) viruses with a neuraminidase substitution (H275Y) (A[H1N1]pdm09) have been identified in Australia, according to a letter published in the Dec. 29 issue of the New England Journal of Medicine.

Aeron C. Hurt, Ph.D., from the World Health Organization Collaborating Center for Reference and Research on Influenza in Melbourne, Australia, and colleagues investigated sustained community transmission of oseltamivir-resistant A(H1N1)pdm09 viruses in Australia. Reverse-transcriptase-polymerase-chain-reaction was performed on 182 patients between May and August 2011 in the Hunter New England region of New South Wales in Australia. Cultures were attempted for all viral isolates; 90 of these were successful.

The investigators identified 29 A(H1N1)pdm09 viruses with the H275Y neuraminidase substitution, all of which were resistant to the adamantine class of antiviral agents. Fifteen of the 90 cultured isolates had the H275Y substitution. The frequency of H275Y variants ranged from 8 percent in June, to 24 percent in July, and 9 percent in August. The resistant strains were closely related in hemagglutinin and neuraminidase sequence analysis (99.9 to 100 percent and 99.6 to 100 percent similarity, respectively). Most of the patients lived within a 50 km radius of Newcastle, with three patients living further away; and two genetically-related strains were identified elsewhere within the state. Only one patient with resistant influenza had previously received oseltamivir. The resistant viruses were antigenically similar to the vaccine strain, and three of the patients had received the 2011 vaccine.

"It remains important to ensure that A(H1N1)pdm09 strains from early in the season are analyzed rapidly for any indication that this transmissible oseltamivir-resistant variant has spread," the authors write.

Letter

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