Age-period-cohort analysis of influenza and pneumonia in US seniors, 1991-2004

The objective of this analysis is to assess pneumonia and influenza patterns in the United States elderly with respect to age, season, and birth cohort. Pneumonia and influenza rates increase with age and vary in intensity by influenza season. Birth cohort has been shown to be related to morbidity and mortality from other diseases and conditions. Older adults experience the highest disease burden and most severe complications from these diseases. All hospitalizations from the Centers for Medicare and Medicaid Services due to pneumonia or influenza (ICD-9CM codes 480-487) were abstracted and categorized by single-year of age, sex, and influenza season. Total counts were then divided by intercensal estimates of age-specific population levels from the US Census to obtain age- and season-specific rates. These rates were log-transformed and linear models were used to assess the relationships among pneumonia and influenza rates and age, influenza season, and birth cohort. In all models, the exponential increase in disease rates with age accounted for most of the variability by age and influenza season. However, after accounting for age and individual influenza season, there were consistent relationships between disease rates and birth cohorts in both males and females. Consistent and positive associations existed between combined pneumonia and influenza rates in the 1898-1900, 1902-03, and 1916-17 birth cohorts. These results suggest that age and birth cohort explain a substantial amount of the variability in age-specific disease rates. Determining the most accurate way of quantifying age and cohort trends merits further research.