we obtained weekly age - specific ili incidence rates during 20042010 from a systematic national surveillance system in chile ( 13 ) . ili surveillance relies on 42 sentinel outpatient sites located throughout the country ; these sites are representative of the general population and systematically report weekly age - specific physician visits for ili ( 13 ) ( online technical appendix , http://wwwnc.cdc.gov/eid/article/20/7/13-0967-techapp1.pdf ) . we characterized the effect of the 2-week winter break period on influenza transmission during 20042010 by comparing trends in weekly ili incidence rates among schoolchildren ( 514 and 1519 years of age ) and adults ( 2064 and > 65 years of age ) . to estimate changes in the age distribution of ili patients , on the basis of methods used in previous work ( 8,14 ) , we compared the weekly ratios of ili incidence rates for schoolchildren and adults during the 2-week period before , during , and after the winter break by using a 1-sided z test . we also considered a 6-week window ( 8,14 ) before and after the winter break as a sensitivity analysis . a decline in the schoolchildren - to - adult incidence rate ratio indicates a shift in the age distribution of patients toward adults , suggestive of decreased influenza transmission among schoolchildren ( 8) . in chile , september , which is typical of temperate regions in the southern hemisphere ( 15 ) . the 2-week winter school break typically coincides with the influenza season and is synchronous throughout the country ; 95% of educational institutions follow the break periods set by the ministry of education . figure 1 illustrates trends in ili incidence rates among schoolchildren 519 years of age and adults > 20 years of age throughout the year and the associated schoolchildren - to - adults incidence rate ratio . in chile , ili incidence displays bimodal patterns of activity ; activity increases before and after the winter break , and transmission is reduced during the break . the schoolchildren - to - adults ratios decreased substantially ( 40%68% ) during the 2-week winter break period relative to the 2-week period immediately preceding the winter break ( table 1 ) . also , the reduction in ratios coinciding with the first week of the winter break occurred every year of our study , including during the pandemic ( 2009 ) and postpandemic ( 2010 ) seasons . average weekly incidence rates for influenza - like illness ( ili ) among schoolchildren 519 years of age and adults > 20 years of age , chile , 20042010 . b ) average ili incidence rate ratio of schoolchildren - to - adult incidence by week . examination of a 2-week period and comparison of the averaged within  week - of - year ili incidence rate ratio for children ( 519 years of age ) to adults ( > 20 years of age ) to the average of the ratio in the 2-week period immediately before provided 50 such comparisons . the bonferroni corrected   =  0.05 ; significance level is thus   =  0.05/50  =  0.001 . the only 2-week periods in which the ratio comparison p value was less than   =  0.001 were the periods beginning week 28 and 29 ( which corresponds to the winter break ) , week 44 ( which corresponds to the reformation / all saints day 4-day weekend ) , week 21 ( the week of the naval glories day break ) , and week 38 ( the independence day break ) . * period begins 2 weeks after the winter break ends because the reduction in the incidence rate ratio during the winter break was maintained on average for 2 weeks after the end of the winter break . p value of a 1-sided z test comparing the average incidence rate ratio ( ratio of incidence rate for schoolchildren to incidence rate for adults ) during the 2-week period before the school break to that during the winter break . small p values indicate that the incidence rate ratio for the period before the break is significantly higher than that for the period during the break;. p values near 1.00 indicate that the incidence rate ratio for the period before the break is significantly lower than that for during the break . p value of a 1-sided z test comparing the average incidence rate ratio during the 2-week period after the school break to that during the winter break period . small p values indicate that the incidence rate ratio for the period after the winter break period is significantly higher than that for the period during the winter break ; p values near 1.00 indicate that the incidence ratio for the period after the break is significantly lower than that for the period during the break . the reduction in the schoolchildren - to - adults incidence rate ratios was maintained for an average of 2 weeks after the end of the winter break . the decline in ratios was primarily caused by a decrease in ili rates among schoolchildren ; the average ( + standard error of the estimate ) reduction in ili incidence among schoolchildren ( 519 years of age ) in the 2 weeks during the winter break compared with the 2 weeks before was 67.2% + 2.1% ( p<0.001 ) . in contrast , the average reduction in adult ili incidence ( > 20 years of age ) was more modest but remained significant at 37.4% + 0.9% ( p<0.001 ) . furthermore , the incidence rate ratios for school - age children to middle - age adults significantly increased after the winter break , signaling a return toward a higher proportion of ili cases among children , although the ratio did not return to prebreak levels ( table 1 ) . in contrast , the ratio comparing rates for children with rates for adults did not change . our results did not change when we used a 6-week period before and after the winter break period instead of a 2-week period ( table 2 ) or when we excluded the 2009 pandemic year from our analysis ( technical appendix 
 figure 1 ) . * period begins 2 weeks after the winter break ends because the reduction in the incidence rate ratio during the winter break was maintained on average for 2 weeks after the end of the winter break . p value of a 1-sided z test comparing the average incidence rate ratio ( ratio of incidence rate for schoolchildren to incidence rate for adults ) during the 2-week period before the school break to that during the winter break . small p values indicate that the incidence rate ratio for the period before the break is significantly higher than that for the period during the break ; p values near 1.00 indicate that the incidence rate ratio for the period before the break is significantly lower than that for during the break . p value of a 1-sided z test comparing the average incidence rate ratio during the 2-week period after the school break to that during the winter break period . small p values indicate that the incidence rate ratio for the period after the winter break period is significantly higher than that for the period during the winter break ; p values near 1.00 indicate that the incidence rate ratio for the period after the break is significantly lower than that for the period during the break . we have shown that a two - thirds decline in ili incidence among schoolchildren coincided with the onset of the school winter break in chile ; this pattern was consistent across the 7 years of the study . in line with a prior study in argentina ( 6 ) , the average reduction in schoolchildren - to - adults incidence rate ratio was sustained for up to 2 weeks after school sessions resumed . this time scale is consistent with the natural history of influenza virus infection , which has a serial interval ( interval between cases ) of 23 days , so that it takes a few successive chains of transmission to reach full - scale transmission . similar to findings from prior studies ( 5,6 ) , our findings are based on analysis of ili incidence , which is a broad indicator of respiratory disease activity in a community and is not entirely specific for influenza . our results could be affected by changes in health - seeking behavior during the winter break . however , our ili data are well correlated with influenza virus activity data ( 15 ) ( figure 2 ) , and large increases in incidence among schoolchildren during winter 2009 coincide with the influenza a(h1n1)pdm09 virus pandemic period , suggesting that fluctuations in ili incidence in chile are primarily attributable to influenza . although the winter break took place near the peak of the 2009 influenza a(h1n1 ) pandemic in chile , it was correlated with changes in the age distribution of patients hospitalized for influenza a(h1n1)pdm09 virus infection ( 7 ) . weekly time series of influenza - like - illness ( ili ) cases and laboratory - confirmed influenza notifications ( flunet ) , in chile , 20042010 . overall , our study findings add to the body of information provided by empirical studies , supporting the implementation of school closure to achieve temporary reductions in ili incidence rates , especially among school - age children , including in the southern hemisphere temperate setting ( 27 ) . our finding that ili incidence was more modestly reduced among adults during winter breaks is consistent with past work on the age - specific transmission dynamics of influenza ( 27 ) . school closure may be particularly useful in pandemic situations to gain time until pharmaceutical measures ( vaccines , antiviral medications ) become available and to mitigate the burden on health care institutions by reducing the surge of influenza patients . there is still , however , little information available from tropical and southern hemisphere settings , which are characterized by complex influenza seasonality patterns and/or low connectivity with the rest of the world and particular demographic and health conditions . systematic multicountry and multiyear comparison of the effects of school closures could shed light on the effectiveness of school - based intervention policies under different epidemiologic , behavioral , and demographic situations . average weekly incidence rates for ili among schoolchildren 515 years of age and adults > 20 years of age , chile , 20042010 , excluding data from the 2009 influenza a(h1n1 ) pandemic year .
to determine effects of school breaks on influenza virus transmission in the southern hemisphere , we analyzed 20042010 influenza - like  illness surveillance data from chile . 
 winter breaks were significantly associated with a two - thirds temporary incidence reduction among schoolchildren , which supports use of school closure to temporarily reduce illness , especially among schoolchildren , in the southern hemisphere .