Two classes of antivirals, the adamantanes and the neuraminidase inhibitors (zanamivir, Relenza; oseltamivir, Tamiflu), are currently approved for the treatment of influenza. The development of resistance to these drugs reduces the options for treating seriously ill patients. Antiviral resistant viruses demonstrate differing ‘fitness’ (the ability to transmit and spread in the community). Widespread transmission of ‘fit’ resistant viruses is of significant public health concern. 

Surveillance for antiviral resistance in influenza is conducted at the WHO Collaborating Centre for Reference and Research on Influenza using in vitro assays. Using a ferret model, the fitness of strains is investigated.

Virtually all currently circulating influenza strains are resistant to the adamantanes, making them unsuitable for the treatment of patients. Fortunately the majority of strains remain sensitive to the neuraminidase inhibitors. However, approximately 2% of A(H1N1)pdm09 strains are resistant to oseltamivir, of which an increasing proportion are being detected from patients not being treated with oseltamivir. In addition, a widespread cluster of cases of oseltamivir-resistant influenza in Newcastle in 2011 further demonstrated the ability of these viruses to spread among the community. Ferret transmission studies indicated that these oseltamivir-resistant viruses are fitter than previous A(H1N1)pdm09 oseltamivir-resistant strains.

The widespread transmission and circulation of oseltamivir resistant A(H1N1)pdm09 viruses remains a risk in the future. Close monitoring of resistant viruses in both treated and community patients remains important.