The recent worldwide spread of pandemic influenza (H1N1) 2009 was notable for its intensity and the demographics of the patients most affected - namely those who were adolescents/young adults, indigenous, obese, or pregnant. Based on serology, approximately 15-20% of the Australian population were affected in 2009, with hospitalisation required in 13% of lab-confirmed cases. Although it is difficult to assess whether, overall, the 2009 pandemic was worse than seasonal influenza, in some regions such as Victoria, the reproduction ratio was sufficiently high (2.4-3.0) to sustain transmission independently. Co-morbidities were important in severe cases - being present in approx. two-thirds of patients requiring hospitalisation or intensive care (asthma 31%; diabetes 18%; immunosuppressive medications 16%; pregnancy 13%; underlying malignancy and obesity). IgG2 subclass deficiency appeared to be associated with severe H1N1 and may partially explain why pregnancy was a risk factor severe H1N1 disease. Unlike seasonal influenza, H1N1 disease was associated with a frequent need for ICU and extra corporeal membrane oxygenation (14% all hospitalised patients required ICU admission - 75% required mechanical ventilation and 14% ECMO).

Subsequently H1N1 has continued to cause problems during the 2010 and 2011 winters in both northern and southern hemispheres, although the proportion of influenza illness due to H1N1 has declined somewhat in 2011, possibly related to the impact of vaccination strategies. Overall, oseltamivir resistance remains uncommon.