Hospitalizations associated with influenza and respiratory syncytial virus in the United States, 1993-2008

Background. Age-specific comparisons of influenza and respiratory syncytial virus (RSV) hospitalization rates can inform prevention efforts, including vaccine development plans. Previous US studies have not estimated jointly the burden of these viruses using similar data sources and over many seasons. Methods. We estimated influenza and RSV hospitalizations in 5 age categories (<1, 1-4, 5-49, 50-64, and ≥65 years) with data for 13 states from 1993-1994 through 2007-2008. For each state and age group, we estimated the contribution of influenza and RSV to hospitalizations for respiratory and circulatory disease by using negative binomial regression models that incorporated weekly influenza and RSV surveillance data as covariates. Results. Mean rates of influenza and RSV hospitalizations were 63.5 (95% confidence interval [CI], 37.5-237) and 55.3 (95% CI, 44.4-107) per 100 000 person-years, respectively. The highest hospitalization rates for influenza were among persons aged ≥65 years (309/100 000; 95% CI, 186-1100) and those aged < 1 year (151/100 000; 95% CI, 151-660). For RSV, children aged < 1 year had the highest hospitalization rate (2350/100 000; 95% CI, 2220-2520) followed by those aged 1-4 years (178/100 000; 95% CI, 155-230). Age-standardized annual rates per 100 000 person-years varied substantially for influenza (33-100) but less for RSV (42-77). Conclusions. Overall US hospitalization rates for influenza and RSV are similar; however, their age-specific burdens differ dramatically. Our estimates are consistent with those from previous studies focusing either on influenza or RSV. Our approach provides robust national comparisons of hospitalizations associated with these 2 viral respiratory pathogens by age group and over time.