We analyzed the impact of the winter vacation period by monitoring temporal patterns in the ratio of incident student (defined as people 5-20 years of age) to non-student influenza A/ H1N1 cases. At the national scale, this ratio was significantly below 1.0 during the winter school vacations and exceeded 1.0 two weeks following the resumption of school activities on August 6 (Wilcoxon test for differences in ratio of student to non-student cases before and during the winter school vacation period, P,0.001, Figure 5 ). At the level of departments, the average ratio of student to non-student influenza A/H1N1 cases decreased by 46% during the 3-week school closure period as compared to the preceding 6 weeks (T-test for differences in mean ratio, P = 0.007, Figure 6 ). Moreover, the ratio was lower and stayed below 1 for longer in the Lima metropolitan area than in the rest of Peru, consistent with the early pandemic wave changing the age structure of susceptible individuals in Lima in June 2009 (Figure G in Text S1).