Our modelling clearly shows that the location of the source case will have a significant effect on the number of infections that occur during a flight ( Figure 2 ). If the plane is completely occupied, a single infectious individual in Economy Class will cause more infections that a source case in First Class ( Figure 2 ). We calculate that H1N1, even during long flights, poses a low to moderate withinflight transmission risk if the source case travels First Class. Specifically, 0-1 infections could occur during a 5 hour flight, 1-3 during an 11 hour flight and 2-5 during a 17 hour flight ( Figure 2 ). However, our results show that within-flight transmission could be significant, particularly during long flights, if the source case travels in Economy Class. Specifically, 2-5 infections could occur during a 5 hour flight, 5-10 during an 11 hour flight and 7-17 during a 17 hour flight (Figure 2 ). More infections would occur in the Economy Class than in Business or First Class because the majority of the passengers travel in Economy and that section has the smallest volume of air per passenger. However, if the aircraft is only partially loaded, under certain conditions, more infections could occur in First Class than in Economy Class. For example, we calculate that, during a 17 hour flight, a greater number of infec-