PROF. FERGUSON: I, too, always thought of influenza having an R0 similar to that of measles and other such diseases. However, a detailed analysis of inter-pandemic flu—particularly household transmission rates—shows that the virus is not that transmissible. The inter-pandemic R0 is probably about the same as the pandemic R0.
The question is what is the R0 of influenza? The study undertaken by Mills et al., and other historical studies of transmissibility based on mortality data, show that whether we measure mortality or infection rates does not matter as long as the ratio of one to the other remains constant. I do not believe the measures are affected by the fact that we are looking at deaths rather than infections.
The one caveat regarding inter-pandemic transmission rates is that multiple strains are circulating. The accumulated reproduction number of all the strains in circulation at a particular point in time is probably somewhat higher than that of a single strain. However, it's probably no more than 3–4, which is considerably less than that of either varicella or measles.