For the 1918 ''Spanish Flu'' H1N1 pandemic, the median growth rate in large US cities was r = 0.20 per day [30, 35] , with comparable estimates in the UK [17] . This value also serves as an upper estimate for the spread of the H2N2 pandemic virus in 1957 [17] . Based on this growth rate, the estimated individual reproduction number is R = 1.74, while the estimated household reproduction number is R * = 2.26, and thus the out-of-household reproduction number is R G = 1.21. Of course, households were bigger in 1918 than now, so that the actual value of R * was likely higher than this. These estimates would imply that a proportion 121/R * = 56% of between household transmission would need to be blocked to prevent epidemic spread. Figure 3 could provide a rough guide to the likely values of R * and R G for a new influenza pandemic where the rate of exponential growth can reliably be determined.