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)).