A key determinant of the success of influenza containment is the transmission rate of the novel strain. C. Fraser et al. [“Pandemic potential of a strain of influenza A (H1N1): Early findings,” Reports, 19 June, p. 1557] estimated the basic reproduction number (R₀) of the Mexican outbreak of influenza A (H1N1) to be in the range of 1.2 to 1.6. The value of R₀ is a key measure of transmissibility and estimates the number of secondary cases in a completely susceptible population. Their findings were comparable to lower estimates for the 1918 pandemic, where R₀ ranged from 2 to 3.