In order to quantify the impact of missing data on the estimation of R 0 and the serial interval, we consider a simulation study. We use multinomial and gamma distributed serial intervals. The multinomial represents a recent estimate for the current Influenza A/H1N1 outbreak in the USA and has a mean, μ of 2.21 days and variance, s 2 of 0.89 with k = 4 [14] . The Gamma distributed serial interval represents a disease with a mean of 8 days and variance of 16 days with k set to 20. We consider three values for R 0 : 1.5, 2, and 2.5, making six simulation scenarios.