Current Challenges in the Risk Assessment of Neuraminidase Inhibitor‐Resistant Influenza Viruses

resistant viruses that retain replicative fitness is a significant obstacle in the effective management of influenza virus infections and poses a threat to pandemic strategies to reduce morbidity and mortality when vaccine is either not available or in limited supply. There is a need to better understand virological and clinical correlates of viral fitness and the methods used to assess viral fitness. The results of previous in vivo studies indicated that mutations conferring resistance to NA inhibitors (NAIs) often lead to a loss of viral fitness [2‐5]. This is in contrast with the results of Baz and colleagues, which suggest that the viral fitness and virulence of oseltamivir-resistant influenza A(H1N1) viruses were unimpaired compared with those of oseltamivir-susceptible viruses. The conclusions drawn by Baz and colleagues are consistent with the available (although somewhat limited) epidemiological information [6‐9]. Nevertheless, it is essential to analyze in more detail the experiments and experimental design that led to these