Influenza Vaccination in Young Children Reduces Influenza-associated Hospitalizations in Older Adults, 2002–2006

Steven A. Cohen, Dr PH; Kenneth K.H. Chui, PhD; Elena N. Naumova, PhD

Disclosures

J Am Geriatr Soc. 2011;59(2):327-332. 

In This Article

Abstract and Introduction

Abstract

Objectives: To assess how influenza vaccination coverage in children is related to pneumonia and influenza (P&I) in older adults and whether sociodemographic factors modify these associations.
Design: Approximately 5 million hospitalization records from the Centers for Medicare and Medicaid Services for four influenza years (2002–2006) were abstracted. A single-year age distribution of rates of P&I hospitalization was estimated according to state for each influenza season; an exponential acceleration in the P&I rates with age was observed for each influenza season. State- and season-specific P&I rate accelerations were regressed against the percentage of vaccinated children, older adults, or both using mixed effects models.
Setting: U.S. population, 2002 to 2006.
Participants: U.S. population aged 65 and older.
Measurements: State-level influenza annual vaccination coverage data in children and older adults were obtained from the National Immunization Survey and the Behavioral Risk Factor Surveillance System, respectively.
Results: Child influenza vaccination coverage was negatively associated with age acceleration in P&I, whereas influenza vaccination in the older adults themselves was not significantly associated with P&I in older adults.
Conclusion: Vaccination of children against influenza may induce herd immunity against influenza for older adults and has the potential to be more beneficial to older adults than the existing policy of preventing influenza by vaccinating older adults themselves.

Introduction

Influenza and associated diseases are among the leading causes of death for older adults in the United States. In the general population, there were nearly 1.7 million annual hospitalizations attributable to pneumonia and influenza (P&I), with 65% of those cases having P&I as the primary diagnosis between the 1979/80 and 2000/01 influenza seasons.[1] Of the 1.7 million annual hospitalizations for influenza-associated diseases, 1.2 million are in the population aged 65 and older. Older adults also experience more-severe complications from these diseases than all other age groups. The P&I mortality rate is 22.1 per 100,000 in the population aged 65 to 99 (hereafter "older population"), nearly 100 times as higher as in the group with the next highest rate (0.3/100,000 in children <1). From 2000 to 2002, there were more than 1.2 million hospitalizations in the older population with any listed diagnosis of pneumonia.[2] The population aged 85 and older experience the highest rates of hospitalization from P&I (629/100,000 person-years).[1] P&I morbidity increases nearly exponentially with age in the older population,[3] probably because of declining immunological function with age.[4] The aging and growth of the older population is to likely magnify the effect of P&I in this vulnerable population.

Influenza is easily spread from person to person. One means of controlling influenza is to vaccinate those individuals who are most likely to spread the infection first, particularly in environments in which vaccination is not obligatory or where vaccine supply is limited. The primary means of controlling influenza, which is largely preventable, is through annual vaccination.[5] Vaccine safety in children has been well documented for inactivated vaccine[6] and live attenuated vaccine[7] in children aged 9 to 17. In children, influenza vaccine has been shown to be highly efficacious in clinical settings, with efficacy ranging from 65% to 96%.[8] For the older population, vaccine efficacy is consistently lower (12–68%[9,10]). Clinical efficacy of the influenza vaccine is higher in older adults younger than 75 than in those aged 75 and older,[11] which is consistent with the observed declines in immune function with age.[12]

Recent research suggests potential pathways for transmission of influenza virus between children and the older population. Influenza vaccination coverage in children was associated with lower all-cause mortality and mortality from P&I in the older population in Japan,[13] in addition to protecting children themselves from death.[14] Mass influenza vaccination of children was shown to significantly reduce influenza-like illness in children and influenza-related diseases in older adults.[15] These results suggest herd immunity, which occurs when vaccinating a substantial portion of the population reduces exposure for those who were not vaccinated,[16] and has been demonstrated in other infectious diseases, including Haemophilus influenzae type b,[17] measles,[18] and oral cholera.[19] A community with customary vaccination practices experienced substantially higher influenza incidence in the adult population than in a neighboring community in which 85% of schoolchildren were vaccinated against influenza.[20] Modestly but significantly lower influenza rates in adults aged 50 to 64 occurred where children in one school were immunized against seasonal influenza than in a control school,[21] suggesting that such a vaccination strategy may impart indirect protection to older adults beyond the contribution of vaccination of older adults themselves.

The objective of this study was to assess the potential for herd immunity in the older population against P&I-associated hospitalization using the most complete database of hospitalization records of older adults. Associations between vaccination coverage in children and P&I hospitalizations in the older population were assessed during 2002 to 2006. The effect of influenza vaccination coverage in the older population on P&I outcomes in the older population was also examined.

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