Time-series analysis of the relation between influenza virus and hospital admissions of the elderly in Ontario, Canada, for pneumonia, chronic lung disease, and congestive heart failure.

This study examined the relation between the presence of circulating influenza virus and all hospital admissions of people over age 65 years in Ontario, Canada, for pneumonia, congestive heart failure, and chronic lung disease in 1988-1993. Autoregressive integrated moving average (ARIMA) transfer function models were used to perform a time-series analysis. These models were compared with simple cross correlations by using Pearson's product moment correlation. The results showed statistically significant correlations between the presence of influenza virus and admissions of the elderly for pneumonia (in all 5 years under study) and chronic lung disease (in 4 of the 5 years under study). The relation between circulation of influenza virus and admissions for congestive heart failure was inconsistent. The simple cross correlation tended to overestimate the association between the presence of circulating influenza strains and hospital admissions. Measures of the impact of influenza should include chronic lung disease as an outcome. Further studies, with greater covariate control, are required to delineate more precisely the relation between influenza and hospital morbidity in the elderly. This study demonstrates the power and utility of using time-series methods in the epidemiologic study of communicable diseases.