Epidemiology of influenza
Influenza is a respiratory infection caused by influenza A and B viruses, which generally occurs in Canada in late fall and in winter. It has a range of illness patterns, from mild cold symptoms and typical flu to life-threatening pneumonia, bacterial infections and other complications.
Influenza is also highly contagious, affecting between 10 and 20 per cent of the world’s population each year (World Health Organization, 2008). Children have the highest infection rates (20-30 per cent), although generally, the most serious rates of illness or death from the flu occurs in people 65 years of age and older and in those with underlying medical conditions (National Advisory Committee on Immunization, 2012).
The majority of people who show symptoms and seek health care are not routinely tested for influenza, due to volumes, test accuracy, timeliness and cost. Although it is the recommended clinical practice, the lack of regular testing makes it difficult to assess the true burden of influenza in terms of incidence, deaths and hospitalizations.
The Public Health Agency of Canada estimates that 2,000 to 8,000 Canadians die of influenza and its complications in a given year, depending on the severity of the flu season (Public Health Agency of Canada, 2012b). Annually, it may be related to as many as 20,000 hospitalizations (Schanzer, Langley, & Tam, 2006; Schanzer, Langley, & Tam, 2008). Statistics Canada (2011) lists it as the seventh most frequent cause of death.