Rates of Influenza-like Illness and

To determine effects of school breaks on influenza vi-rus transmission in the Southern Hemisphere, we analyzed 2004–2010 influenza-like–illness surveillance data from Chile. Winter breaks were significantly associated with a two-thirds temporary incidence reduction among school-children, which supports use of school closure to temporar-ily reduce illness, especially among schoolchildren, in the Southern Hemisphere. Influenza pandemic preparedness plans to mitigate effects of a severe pandemic recommend layered medical and social distancing interventions, including school closings, cancellation of large public gatherings, and face mask use (1). Because schoolchildren are considered to be high trans-mitters of influenza virus (higher contact rates, enhanced susceptibility to infection, and increased virus shedding relative to that among persons in other age groups), prompt school closure is expected to reduce transmission during a pandemic (2). Although several empirical studies have linked school activities with influenza virus transmission (2–10), few studies have considered data from multiple epidemic pe-riods, and little information is available from the Southern Hemisphere. School breaks and school teacher strikes pro-vide natural experiments in which the effect of school terms on influenza transmission dynamics can be explored. On the basis of 21 years of surveillance data, Cauchemez et al. (5) found a 16%–18 % reduction in incidence of influenza-like illness (ILI) associated with the 2-week school winter break periods in France. A study of variation in contact rate patterns in Europe suggested a 13%–40 % reduction in the basic reproduction number associated with school breaks in Belgium, Great Britain, and the Netherlands (11). A 12-day teacher strike in Israel in the winter of 1999 was also asso-ciated with a reduction (43%) in weekly rates of respiratory disease (12). A single study is available from the Southern Hemisphere and indicates a 14 % reduction in ILI incidence during winter break in Argentina during 2005–2008; the largest decrease was observed among children 5–14 years of age (6). In our study, we quantified the effect of school break cycles on the age distribution of ILI patients in Chile during 2004–2010