"In April 2009 the World Health Organization declared a formal “public health emergency of international concern,” marking the start of an international public health response to the first influenza pandemic of the 21st Century. One of the immediate priorities was to quantify the transmissibility of the new pandemic influenza A (H1N1pdm09) virus (denoted H1N1pdm09 hereafter) and the seriousness of infection with this virus, because these two epidemiologic measures in combination determine the severity of the pandemic in the absence of control measures. Whereas a number of transmissibility estimates, based on the reproduction number R, were published with broad agreement from the early stages of the pandemic, there was far greater difficulty in estimating the seriousness of infections. In the report of the World Health Organization’s Review Committee on the functioning of the 2005 International Health Regulations in relation to H1N1pdm09, Fineberg et al. identified “the absence of a consistent, measurable and understandable depiction of severity of the pandemic” as one of the major shortcomings of the international public health response."