Another important consideration in setting out to provide mass treatment and prophylaxis with NAIs is the possibility of development of resistant strains. Surveillance studies during the first wave of the pandemic demonstrated a low frequency of resistance to oseltamivir and no reported resistance to zanamivir. Prior to 2007, it was rare to detect oseltamivir-resistant influenza strains in untreated patients, due to the compromised infectivity and transmissibility of many of the resistant mutants in the absence of drug pressure. But in 2007/2008 an oseltamivir-resistant seasonal A(H1N1) variant emerged that demonstrated viral fitness at least equivalent to the oseltamivir-susceptible strain. The resistant strain spread rapidly around the world and by 2009 had completely replaced the susceptible strain [41] . An oseltamivir-resistant pH1N1 virus might also retain viral fitness and subsequently spread throughout the community. Fortunately, to date, only a low frequency of oseltamivir-resistant pH1N1 strains have been identified [42] .