We assumed a mean generation interval of three (1/b 1 = 1.5 days and 1/b 2 = 1.5 days) and four days (1/b 1 = 2 days and 1/b 2 = 2 days), which are within the range of mean estimates for the 2009 influenza pandemic [29, 30, 31, 32] . We assessed the sensitivity of our estimates to small variations in the definition of the ascending phase used to estimate the exponential growth rate (+/24 days).