Data
Allegheny County Health Department (ACHD) provided
virologically-confirmed influenza case data (N=11,946) for 2007-2008,
and 2010-2015 influenza seasons. A reported confirmed influenza case was
a positive laboratory-confirmed test (i.e., rapid diagnostic test,
reverse-transcriptase polymerase chain reaction, or viral culture)
reported by hospital emergency departments or sentinel medical providers
in Allegheny Country from any individual experiencing influenza-like
illness. Weekly all-cause absences and school enrollment data for
2010-2015 came from nine Allegheny County school districts. Six
districts provided grade-specific absences (Supplemental Text and
Supplemental Table 1). Grade-level served as an age-proxy, since student
demographics (i.e., age, gender, or vaccination status) were
unavailable. Additional all-cause and cause-specific absences came from
three school-based cohort studies (Pittsburgh Influenza Prevention
Project (PIPP) during the 2007-2008 season (10 K-5 schools), Social
Mixing and Respiratory Transmission in School study (SMART) during the
2012-2013 season (eight K-12 schools), and Surveillance Monitoring of
Absences and Respiratory Transmission (SMART2) study
during the 2015-2016 season (nine K-12 schools)). Cohort studies used
similar absence collection protocols(11).
Greater Pittsburgh area daily minimum and maximum temperature and
relative humidity data came from the National Oceanographic and
Atmospheric Administration’s National Climatic Data Center(12). We used
temperature and relative humidity (a proxy for absolute humidity) given
their effects on influenza transmission (i.e., viral dispersal and
survival)(13, 14). Allegheny County population data came from US Census
Bureau’s yearly estimates for 2007, 2008, and 2010-2015(15).
Our primary outcome was weekly confirmed influenza infections reported
in Allegheny County during 2007-2008 and 2010-2015 seasons. Influenza
infection was defined as any virologically-confirmed case reported by a
health provider in Allegheny County during CDC-defined influenza seasons
(i.e., 40th calendar week to the
20th calendar week of the subsequent year)(16) Weekly
influenza cases were the total cases reported each week, excluding cases
occurring during school closures (e.g., spring break, federal holidays,
weekends).
All-cause absences were defined as a full or partial school day missed
for any reason. Cause-specific absences were a full or partial school
day missed due to influenza-like illness (i.e., fever
(>37C) and either cough, sore throat, runny nose, or
congestion). We restricted school absences to periods overlapping the
influenza seasons to examine absence patterns during influenza
circulation, and excluded weekends, observed federal holidays, and
school breaks. Weekly school absences were the total absences reported
in one school week (i.e., if no observed holidays, five days in a school
week). Weekly absence rates were total absences in a week, divided by
the total students enrolled times the number of school days in a given
week.
Daily average minimum and maximum temperatures and relative humidity
were used to estimate weekly average temperature and relative humidity
for each influenza season.