Influenza has been a reportable disease in Ontario since 1923. A highly contagious acute viral disease of the respiratory tract, influenza continues to be a major cause of preventable morbidity and mortality in Ontario. Influenza viruses are divided into three types, designated A, B, and C. Influenza types A and B are responsible for epidemics of respiratory illness. It is estimated that annually, influenza and community-acquired pneumonia account for 60,000 hospitalizations and 8,000 deaths most of which occur among the elderly persons¹. Groups at increased risk for influenza complications include the elderly, immunocompromised and persons with chronic medical conditions. In Canada, on average, 500-1,500 deaths every year are due to influenza alone. Annual attack rates in Canada range from 10 to 20% each year and can be considerably higher in epidemics. Current control measures in Canada include vaccination and chemoprophylaxis with anti-virals. The antigenic characteristics of the circulating strains provide the basis for selecting the strains that are included in the annual influenza vaccine.

Health Canada Influenza like illness (ILI) definitions for the 2003-2004 season ²

1. Adult (general population) : Acute onset of respiratory illness with fever and cough and one or more of the following : sore throat, arthralgia, myalgia or prostration, which could be due to the influenza virus.
2. Elderly : Acute onset of respiratory illness with cough and one or more of the following : sore throat, arthralgia, myalgia and prostration. Often fever or chills are present, but these symptoms may not be prominent in the elderly.
3. Pediatric : Acute respiratory illness with cough and fever and one or more of the following : sore throat, arthralgia, myalgia and prostration. In children under 5 years of age, gastrointestinal symptoms may also be present.
4. In patients under 5 years or those who are 65 years and older, fever may not be prominent.

2 = Sporadic : sporadically occurring ILI and confirmed influenza* with NO outbreaks detected within the influenza surveillance region†

3 = Localized : sporadically occurring ILI and confirmed influenza* together with outbreaks of ILI in schools and worksites or laboratory confirmed influenza in residential institutions occurring in less than 50% of the influenza surveillance region(s)†

4 = Widespread : sporadically occurring ILI and confirmed influenza* together with outbreaks of ILI in schools and worksites or laboratory confirmed influenza in residential institutions occurring in greater than or equal to 50% of the influenza surveillance region(s)†

1. Schools and work sites : greater than 10% absenteeism on any day most likely due to ILI.
2. Residential institutions : two or more cases of ILI within a seven-day period, including at least one laboratory confirmed case. Institutional outbreaks should be reported within 24 hours of identification.

* confirmation of influenza within the surveillance region (health unit) at any time within the prior four weeks
† sub-regions within the province as defined by the provincial/territorial epidemiologist (Physician Manager, Disease Control Service for Ontario)

¹ CIHI (Canadian Institute for Health Information – Respiratory Disease in Canada , October 2001

²  Health Canada

Canada Communicable Disease Report (CCDR) Vol. 29, 15 Aug. 2003. The National Advisory Committee on Immunization (NACI) : Statement on Influenza Vaccination for the 2003-2004 Season.

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