Abstract
BackgroundThe optimal vaccination strategy to mitigate the impact of influenza epidemics is unclear. In 2005, a countywide school-based influenza vaccination campaign was launched in Knox County, Tennessee (population 385,899). Approximately 41% and 48% of eligible county children aged 5–17 years were immunized with live attenuated influenza vaccine before the 2005–2006 and 2006–2007 influenza seasons, respectively. We sought to determine the population impact of this campaign.MethodsLaboratory-confirmed influenza data defined influenza seasons. We calculated the incidence of medically attended acute respiratory illness attributable to influenza in Knox and Knox-surrounding counties (concurrent controls) during consecutive seasons (5 precampaign and 2 campaign seasons) using negative binomial regression and rate difference methods. Age-stratified analyses compared the incidence of emergency department (ED) visits and hospitalizations attributable to influenza.ResultsDuring precampaign seasons, estimated ED visit rates attributable to influenza were 12.39 (95% CI: 10.34–14.44) per 1000 Knox children aged 5–17 years and similar in Knox-surrounding counties. During the campaign seasons, annual Knox influenza-associated ED visit rates declined relative to rates in Knox-surrounding counties: rate ratios 0.55 (95% CI: 0.27–0.83) and 0.70 (95% CI: 0.56–0.84) for the first and second campaign seasons, respectively. Overall, there were about 35% or 4.86 per 1000 fewer influenza-associated ED visits among Knox County children aged 5–17 years attributable to the campaign. No significant declines in Knox compared to surrounding counties were detected for influenza associated ED visits in children aged <5 years, all adults combined or selected adult age subgroups, although power for these analyses was limited. Alternate rate-difference analyses yielded consistent results.ConclusionVaccination of approximately 45% of Knox school-aged children with influenza vaccine was associated with a 35% annual reduction (4.86 per 1000) in ED visit rates attributable to influenza. Higher vaccination coverage and/or larger studies would be needed to determine whether similar interventions have indirect benefits in other age groups.
Highlights
The optimal strategy to mitigate the population health impact of influenza epidemics is unclear
Each influenza season began the week when the cumulative proportion of positive rapid influenza tests observed during that winter (November through April) reached 2.5%, and ended the week the cumulative proportion reached 97.5%
Healthcare encounters data We identified healthcare encounters using the electronic Tennessee Hospital Discharge Data System, which includes data on hospitalizations and emergency department (ED) visits from all Tennessee Department of Health-licensed hospitals
Summary
The optimal strategy to mitigate the population health impact of influenza epidemics is unclear. Live attenuated influenza vaccines (LAIVs) are attractive for school-based interventions because compared to inactivated vaccines, they have higher efficacy in young children,[10,11,12] result in reduced virus shedding among vaccinees after influenza infection,[13] and their ‘‘needle-less’’ mode of administration has high acceptability.[11,14,15] School-based vaccination campaigns with LAIV have been reported to reduce influenza illness among vaccinees,[16,17,18] reduce influenza-like illnesses among their close contacts, and modestly reduce school absenteeism.[16,17] mathematical models and experimental evidence suggest benefits of such targeted vaccination strategies among unvaccinated persons in selected populations (i.e. indirect or herd protection), [9,19,20,21,22,23] consistent evidence of herd protection is limited.[14,16,24]. We sought to determine the population impact of this campaign
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