Weekly incidence of influenza cases in Japan from 2009-10. The vertical axis represents the estimated weekly number of cases based on a nationwide sentinel surveillance, covering the period from week 27 (the week ending on 5 July 2009) to week 18 (the week ending on 9 May 2010). The estimates, based on the notified number of cases from a total of 4800 randomly sampled sentinel hospitals, are extrapolated to the total number of medical facilities in Japan. The case represents all influenza-like illness cases that received medical attendance. During the period of interest, influenza A (H1N1-2009) dominated all influenza viruses that were isolated. The four arrows indicate the weeks (weeks 42, 45, 48 and 51 in 2009) that were used for the model predictions in the present study.