The incidence of ICU admission per million inhabitants was higher in Australia than in New Zealand in 2009 (36.5 versus 28.3 admissions to ICU per million inhabitants) [3] . Conversely, in 2010 this incidence was higher in New Zealand. This difference is concordant with national data reporting 6,064 confirmed H1N1 cases in Australia (272 cases per million inhabitants) versus 1,810 in New Zealand (415 cases per million inhabitants in 2010) during the same period [2, 24] . The proportion of confirmed cases hospitalised (727 of 1,810 = 40.1%) [25] , and of hospitalised cases admitted to ICU (82 of 732 = 11.2%) in New Zealand was similar to 2009 [26] . Accordingly, the higher ICU admission rate per population observed in 2010 was due to a higher incidence of community infection, rather than a difference in severity of disease. The reason for the difference in infection rate between Australia and New Zealand in 2010 is unclear, as a similar proportion of the population was seropositive after the 2009 pandemic wave in Australia (22% (95% CI 19.1 to 24.9)) compared to New Zealand (26.7% (95% CI 22.6 to 29.4)) [27, 28] and a similar proportion were vaccinated (21.8% versus 24.1%, respectively) during the inter-wave period. Possible explanations include natural variations in community spread of influenza or an effect of the difference in vaccination deployment where Australia used both early monovalent and later polyvalent vaccines while New Zealand relied predominantly on delivery of the polyvalent vaccine alone, although overall coverage was similar.