We compiled age-specific time series of ILI and A/H1N1 influenza cases by day of symptom onset for May-December 2009 at the refined spatial scale of provinces (N = 134), as well as the coarser scale of departments (N = 24) (Figure 1 ). For each spatial unit we recorded the cumulative number of cases and peak day, defined as the day with the maximum number of new cases. We also estimated the day of pandemic onset defined as the first day of the period of monotonously increasing cases leading up to the peak of A/H1N1 cases, as in [23] . We then investigated geographic variation in the timing of pandemic onset across departments and provinces and their association with demographic factors and distance from Lima.