Characteristics of influenza and school absences
Over seven influenza seasons, 11,946 confirmed influenza cases were reported to ACHD (Supplemental Table 1). Influenza type A predominated most seasons, similar to national patterns(19-24). Overall, 9,350 type A (1,397 A/H3N2 and 1,115 A/H1N1 subtypes) cases, 2,453 type B cases and 143 un-typed cases were reported. The 2011-2012 and 2014-2015 seasons were the lowest (301 cases) and highest (3,150 cases) transmission seasons in Allegheny Country, like national trends. Within seasons, cases peaked in the winter whereas county-level absences varied throughout the year (Figure 1).
During the 2010-2015 seasons, county school districts reported 2,184,200 total absences (Figure1), averaging 6.5 weekly absences/100 students (interquartile range [IQR: 5.6, 7.7]) (Supplemental Table 1). High schools had the highest average absence rates (9.4 weekly absences/100 students, IQR: 8.1,10.8), followed by middle schools (6.3 weekly absences/100 students, IQR: 5.4, 7.7) and elementary schools (5.3 weekly absences/100 students, IQR: 4.0, 6.6). Study schools reported 20,128 all-cause and 3,012 ILI-specific absences among 11,660 students (Supplemental Table 1). The SMART2 study had the highest average weekly all-cause absence rates (2.2 weekly absences/100 students (IQR: 1.8, 2.5)), while the SMART study had the highest ILI-specific absence rates (1.1 weekly absences/100 students (IQR: 0.7, 1.4)).