Imported vs. indigenous H1N1 cases and age-specific travel statistics for various countries. (A) Fraction of indigenous cases and of imported cases during the initial phase of the H1N1 pandemic outbreak in the [0–19] years age class, calculated from surveillance data for the following countries: The Netherlands , Belgium , UK , France , Japan , Italy . (B) Percentage of air-travel passengers in the younger age classes for a set of airports around the world. The age classification used by the demographic statistics vary across countries (Helsinki1, Finland; Teheran3, Iran; Los Angeles2, USA; Amsterdam5, The Netherlands; Heathrow4, Gatwick4, Stansted4, Luton4, UK; Venice6, Italy; Hannover7, Frankfurt8, Hamburg8, Munich8, Germany) with the corresponding age brackets for the children class (expressed in years): 1=[0,15]; 2=[0,18]; 3=[0,19]; 4=[0,20]; 5=[0,21]; 6=[0,25]; 7=[0,26]; 8=[0,30]. Sources of the data are reported in the Additional file 1. The statistics found for Italy and Germany correspond to larger age brackets. If we rescale the data as indicated in the Demographic and travel data subsection, we obtain the following estimates for the percentage of travelers in the [0–18] years old class: 1.05% (Venice), 0.49% (Hannover), 2.31% (Frankfurt), 2.31% (Hamburg), and 2.86% (Munich). These estimated values are consistent with the data.