Pandemic (H1N1) 2009 influenza ("swine flu") in Australian and New Zealand intensive care.

being due to influenza A on the basis of the relationship between number of weekly laboratory confirmations of influenza A and number of weekly deaths. Two factors will ultimately determine the total mortality from H1N1 09 influenza: the attack rate (proportion of the population with symptomatic infection), and the case-fatality rate (proportion of patients with symptomatic infection who die). Predictions from the Australian Government are for an attack rate of 20% and a case-fatality rate of 0.14%, similar to that of seasonal flu. 6 This implies about 6000 deaths in Australia due to H1N1 09. While the epidemic has not yet run its course, it now appears that both the attack rate and case-fatality rate will be substantially lower than this initial estimate. The availability of a vaccine in Australia in September or October is expected to minimise the attack rate. A major difference between seasonal influenza A and H1N1 09 influenza is the age distribution of life-threatening cases and deaths. According to media reports, most deaths from H1N1 09 influenza are in young and middleaged adults. In contrast, most deaths from seasonal influenza A occur in the elderly, 7 and deaths of young people due to seasonal influenza are rare. In the United States, modelling indicates that the annual mortality from influenza A for individuals between the ages of 1 and 49 years is 0.2 per 100 000 person-years. 7 If this were similar in Australia and New Zealand, it would translate to about 30 deaths per year in this age bracket. The true public health burden of H1N1 09 influenza should not be measured by the number of deaths but by the life-years lost.