We used two complementary approaches to assess the association between the transmission dynamics of the pandemic in Peru and the 2-week winter student vacation period, which started after the peak of the pandemic on July 16 and ended on August 6, 2009. First, we evaluated weekly trends in the ratio of incident A/H1N1 cases among the student population (ages 5-20 years) to incident cases among all other age groups in the Lima metropolitan area and the rest of Peru. A decline in the ratio of school-age to other cases is suggestive of an impact of school closure on transmission of pandemic influenza. However this approach does not take into account the natural dynamics of the pandemic and rapid depletion of susceptibles occurring in the first weeks of the outbreak, especially among highly connected individuals such as school-age children. To take into account susceptible depletion, we used a second approach based on an agestructured mathematical model of influenza transmission, tailored to the epidemiology of pandemic influenza and the population of Peru (Text S1). We quantified the expected reduction in the final pandemic attack rate as a function of the reduction of the transmission rate among the student population associated with school closing, the timing of the school closing period in relation to the pandemic peak, and R 0 values in the range 1.6-1.8 [29] .