Using Equations (2-3) we obtain R 0 = 1.0001, just above unity. However, we note that it is more appropriately the reproduction number R of the winter flu epidemic, given the effect of pre-epidemic immunity that must exist. Chowell et al. [31] used several weekly seasonal flu mortality data, derived from P&I excess deaths and influenza-specific deaths from US, France, and Australia during 1972-1997 (1972-2002 for US), to estimate the (mean) reproduction number R p over 3 decades of seasonal flu. They found that the mean of R p to be around 1.3, with year-to-year variability of 0.9-2.1. Our estimate is lower, but within their range.