Rates of Influenza-like Illness and Winter School Breaks
Rates of Influenza-like Illness and Winter School Breaks
We obtained weekly age-specific ILI incidence rates during 2004–2010 from a systematic national surveillance system in Chile. 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 (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 2004–2010 by comparing trends in weekly ILI incidence rates among schoolchildren (5–14 and 15–19 years of age) and adults (20–64 and >65 years of age). To estimate changes in the age distribution of ILI patients, on the basis of methods used in previous work, 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 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.
In Chile, wintertime influenza activity peaks during May–September, which is typical of temperate regions in the Southern Hemisphere. 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 5–19 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.
(Enlarge Image)
Figure 1.
Average weekly incidence rates for influenza-like illness (ILI) among schoolchildren 5–19 years of age and adults >20 years of age, Chile, 2004–2010Error bars represent the standard errors of the mean within each week. The shaded area represents the period of the 2-week winter break. A) Average ILI incidence per 100,000 population, by week. 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 (5–19 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 α = 0.05; significance level is thus α = 0.05/50 = 0.001. The only 2-week periods in which the ratio comparison p value was less than α = 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).
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 (5–19 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). This reduction occurred systematically in each winter of the study period. 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 (online Technical Appendix Figure 1 http://wwwnc.cdc.gov/EID/article/20/7/13-0967-Techapp1.pdf).
The Study
We obtained weekly age-specific ILI incidence rates during 2004–2010 from a systematic national surveillance system in Chile. 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 (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 2004–2010 by comparing trends in weekly ILI incidence rates among schoolchildren (5–14 and 15–19 years of age) and adults (20–64 and >65 years of age). To estimate changes in the age distribution of ILI patients, on the basis of methods used in previous work, 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 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.
In Chile, wintertime influenza activity peaks during May–September, which is typical of temperate regions in the Southern Hemisphere. 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 5–19 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.
(Enlarge Image)
Figure 1.
Average weekly incidence rates for influenza-like illness (ILI) among schoolchildren 5–19 years of age and adults >20 years of age, Chile, 2004–2010Error bars represent the standard errors of the mean within each week. The shaded area represents the period of the 2-week winter break. A) Average ILI incidence per 100,000 population, by week. 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 (5–19 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 α = 0.05; significance level is thus α = 0.05/50 = 0.001. The only 2-week periods in which the ratio comparison p value was less than α = 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).
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 (5–19 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). This reduction occurred systematically in each winter of the study period. 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 (online Technical Appendix Figure 1 http://wwwnc.cdc.gov/EID/article/20/7/13-0967-Techapp1.pdf).
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