While a previous study provided preliminary estimates of the reproduction number during the initial pandemic phase in Peru using an early series of confirmed A/H1N1 cases [35] , our study is based on a consolidated dataset of the pandemic with high spatial and temporal resolution. Our R estimate for the first wave in the greater Lima metropolitan area was estimated in the range 1.7-2.2. Estimates of the reproduction number for the second wave concentrated in other regions of Peru were significantly lower than those obtained for the first wave in Lima (R,1.3-1.5). The higher reproduction number of the first wave in Lima could be explained by the high contact rates characteristic of school age children, the most affected age group during this pandemic wave. Lower population density in other regions of Peru compared to the greater Lima metropolitan area could also support a lower reproduction number associated with the second pandemic wave. A recent review of reproduction number estimates for the 2009 A/ H1N1 influenza pandemic in 20 countries reported a range of R between 1.1 and 3.1 with a median value of 1.6 [36] . R estimates have been reported in the range of 1.2-2.4 for community-based settings in Mexico [29, 34, 37, 38] , Japan [39] , New Zealand [40] , Australia [41] , Chile [42] , Ontario, Canada [43] , and the United States [44] , while higher estimates ranging from 2.3 to 3.3 have been obtained during school outbreaks [6, 30, 45] . The variability in published estimates could be attributed to differences in estimation methods and assumptions, including different generation time distributions, inclusion of correction factors to adjust for case underreporting, and differences in the identification of the growth phase period [36] . Heterogeneity in the timing and intensity of intervention strategies, school activity periods, and climatic conditions [11, 24, 34] could have also contributed to differences in reproduction number estimates across locations. We assessed the correlation of the timing of pandemic onset across spatial units as a function of population size, population density and absolute humidity levels. Our results indicate that areas with larger populations experienced earlier pandemic onset at the scale of the 24 Peruvian departments. This observation suggests a hierarchical dissemination pattern of the pandemic driven by large population centers of Peru, reminiscent of the 1918-1919 influenza pandemic in England and Wales [23] and seasonal influenza epidemics in the United States [46] . While absolute humidity has been found to be significantly associated with the onset of seasonal and pandemic influenza epidemics in the United States [11, 12] , we did not find a significant correlation at the province level in Peru. Further analysis of the environmental or social factors influencing the transmission of seasonal and pandemic influenza is warranted in order to fully explain these patterns [47] .