BACKGROUND:
Influenza is responsible for substantial morbidity and mortality every year in the United States. Burden of disease estimates are critical to influenza vaccine policy formulation. It is important to update these estimates periodically and reexamine the distribution across age and comorbidity status.

OBJECTIVE:
To estimate costs of influenza episodes involving inpatient admissions and examine determinants of costs.

METHOD:
We used Medstat Marketscan 2001 data to examine experiences of 3,737 inpatients diagnosed with influenza-like-illnesses, including influenza, pneumonia, chronic obstructive pulmonary disease, and bronchitis. Patients were categorized by age and comorbidity and followed from 30 days pre-admission through 30 days post-discharge. Costs were captured for all inpatient, outpatient, and pharmaceutical claims. We examined determinants of cost per case including age and comorbidity status, which defined risk of serious complications from influenza.

RESULT:
Costs of inpatient episodes of influenza varied substantially by comorbidity status within age groups. For example, non-comorbid individuals less than age 2 averaged inpatient costs of $13,700 for six day stay with total payments of $16,000, including outpatient and pharmacy. For same-age comorbid individuals, mean inpatient payments were $162,000 for 41 day mean stay with total payments of $172,000. Similarly, for non-comorbid individuals aged 2-17, inpatient payments averaged $11,000 for 6 day stay. For same-age comorbid individuals, mean inpatient payments were $65,300 for 14 day stay. For non-comorbid individuals aged 18-49, inpatient payments averaged $22,300 for 10 day stay. For same-age comorbid individuals, mean inpatient payments were $47,900 for 21 day stay. For non-comorbid individuals over age 50, inpatient payments averaged $28,200 for 13 day stay. For same-age comorbid individuals, mean inpatient payments were $41,000 for 17 day stay. For all age groups, the cost differential between moderate and severe influenza cases (includes neurological complications) was even more dramatic.

CONCLUSION:
Results of this study highlight substantial variation in cost per hospitalized case of influenza. Comorbidities play an important role determining costs of inpatient episodes, particularly among young children. Further analysis is needed to fully describe factors driving these costs.

LEARNING OBJECTIVES: