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.