"In preparation for the next influenza pandemic, it is essential to reach a consensus on how to define and measure the seriousness of infection (an important indicator of the severity of the pandemic), and whether the analysis can be based entirely on estimates of age-specific risk of death among cases. The consistent estimates of the infection fatality risk at around 1 to 10 deaths per 100,000 infections identified in our review may represent the seriousness of H1N1pdm09 in developed countries where data were available. Similar estimates for seasonal influenza viruses, however, are not available for comparison, and neither are estimates from less developed countries in which the seriousness profile would likely be higher."