Background Pneumonia and influenza (P&I) is a major cause of morbidity and mortality in the USA, particularly in elderly people. Recent research indicates that P&I may be linked to socioeconomic conditions associated with interactions of children with vulnerable elderly people that may proliferate the spread of disease. This study assessed the associations between four sociodemographic characteristics—median county income, Gini index, youth dependency ratio and proportion of co-residential caregiver grandparents—and P&I on the county level overall and by age group.

Methods All hospitalisations due to P&I from 1991 to 2004 were abstracted from the Centers for Medicare and Medicaid Services database and categorised by influenza year (July–June) and age category. Using generalised estimating equations, associations between P&I rates and four sociodemographic variables were assessed and models were stratified by income to assess income as a potential effect modifier.

Results P&I rates were higher in counties with lower median income. In low-income counties, high levels of live-in grandparental caregivers were associated with consistently higher levels of pneumonia and influenza rates. The Gini index was positively associated with disease rates, particularly in younger age groups.

Discussion These results suggest complex relationships between sociodemographic characteristics and P&I outcomes for elderly people, particularly those related to children. The strength of the relationship between the proportion of grandparental caregivers and disease rates decreases with age, which may caregiving patterns, or may serve as a proxy for related sociodemographic characteristics. These findings merit further research to understand better how area-level factors affect P&I patterns in elderly people.