The burden of influenza in young children, 2004-2009

Abstract

Objective: To characterize the health care burden of influenza from 2004 through 2009, years when influenza vaccine recommendations were expanded to all children aged ≥6 months.

Methods: Population-based surveillance for laboratory-confirmed influenza was performed among children aged <5 years presenting with fever and/or acute respiratory illness to inpatient and outpatient settings during 5 influenza seasons in 3 US counties. Enrolled children had nasal/throat swabs tested for influenza by reverse transcriptase-polymerase chain reaction and their medical records reviewed. Rates of influenza hospitalizations per 1000 population and proportions of outpatients (emergency department and clinic) with influenza were computed.

Results: The study population comprised 2970, 2698, and 2920 children from inpatient, emergency department, and clinic settings, respectively. The single-season influenza hospitalization rates were 0.4 to 1.0 per 1000 children aged <5 years and highest for infants <6 months. The proportion of outpatient children with influenza ranged from 10% to 25% annually. Among children hospitalized with influenza, 58% had physician-ordered influenza testing, 35% had discharge diagnoses of influenza, and 2% received antiviral medication. Among outpatients with influenza, 7% were tested for influenza, 7% were diagnosed with influenza, and <1% had antiviral treatment. Throughout the 5 study seasons, <45% of influenza-negative children ≥6 months were fully vaccinated against influenza.

Conclusions: Despite expanded vaccination recommendations, many children are insufficiently vaccinated, and substantial influenza burden remains. Antiviral use was low. Future studies need to evaluate trends in use of vaccine and antiviral agents and their impact on disease burden and identify strategies to prevent influenza in young infants.

FIGURE 1

Study population over 5 consecutive influenza seasons (November–April) from 2004 through 2009.

FIGURE 2

Influenza hospitalization rates per 1000 children <5 years of age with any influenza, influenza A, or influenza B, by influenza season (November–April). Line bars show the 95% confidence intervals for each rate.

FIGURE 3

Proportion of outpatient (ED and clinic) children <5 years of age with fever or acute respiratory illness who had any influenza, influenza A, or influenza B during the peak 13 weeks, by season. Line bars show the 95% confidence intervals for each proportion.

FIGURE 4

Proportion of influenza-negative children ≥6 months and <5 years of age who were (A) fully vaccinated or (B) received any vaccine against influenza, by influenza season (November–April) and setting. * Significant increase (P < .001) over time by Cochran-Armitage test for trend.