Comparison of the Epidemiology and Clinical Outcomes of Laboratory-Confirmed Influenza A and Influenza B Cases in Manitoba, Canada

Annual epidemics of influenza are an important public health problem globally and in Canada. Each year, 10-25% of the Canadian population become infected with influenza. Most of these infections are typically asymptomatic or associated with a mild self-limiting illness. However, influenza can cause severe illness leading to hospitalization and death, especially among the very young, the elderly and among those with underlying chronic conditions. It has been estimated that on average about 4,000 influenza-related deaths occur in Canada each year. In addition, the economic burden of influenza is significant because of the high direct costs associated with increased utilization of services and the indirect costs caused by school and work absenteeism. This project compares laboratory-confirmed cases of influenza A and B in terms of their epidemiology, and the overall and cause-specific incidence rates of important clinical outcomes including physician visits, hospitalization and death following clinical diagnosis of influenza cases in Manitoba over the period from 1993 to 2008. The study assesses secular and seasonal trends in the incidence rates of influenza A and B cases in Manitoba during the period of 1993-2008, to determine whether these rates vary for different subsets of the population defined by demographic (e.g., age group, gender), socioeconomic, geographic (neighborhood of residence) and clinical (e.g., vaccination, presence of chronic conditions, immune status) characteristics. The analysis by influenza type then compares the overall and cause-specific incidence rates of physician visits, hospitalization and death following the diagnosis of a laboratory-confirmed influenza. The effects of influenza type on clinical outcomes (e.g., hospitalization) are assessed with multivariate models while accounting for potential confounding by demographic, socioeconomic, geographic and clinical (e.g., co-morbidity, immune status) characteristics.