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 elderly persons¹. Groups at increased risk for influenza complications include the elderly, immuno-compromised 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 per cent 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.

• No Data : No activity report corresponding to the surveillance week was received at the Ministry of Health and Long-Term Care Call Centre by the Tuesday (at 4 p.m.) following the end of the surveillance period.
• No Activity : No laboratory confirmed* influenza cases(s) or outbreak detected during the past four weekswithin the influenza surveillance region, however, sporadically occurring ILI may be reported†
• Sporadic : Sporadically occurring ILI and laboratory confirmed influenza* case(s) with NO outbreaks detected within the influenza surveillance region†
• Localized : sporadically occurring ILI and laboratory confirmed influenza* case(s) together with outbreaks of ILI in schools and/or worksites or laboratory confirmed influenza in residential institutions occurring in less than 50% of the influenza surveillance region(s)†
• Widespread : sporadically occurring ILI and laboratory confirmed influenza* case(s) together with outbreaks of ILI in schools and/or worksites or laboratory confirmed influenza in residential institutions occurring in greater than or equal to 50% of the influenza surveillance region(s)†

*Confirmation of influenza within the surveillance region at any time within the prior four weeks
†Sub-regions within the province or territory as defined by the provincial/territorial epidemiologist.

A) ILI in the general population :
Acute onset of respiratory illness with fever and cough, and with one or more of the following-- sore throat, arthralgia, myalgia, or prostration which could be due to influenza virus. In children under 5, gastrointestinal symptoms may also be present. In patients under 5 or 65 and older, fever may not be prominent.

B) ILI/Influenza outbreaks:
Schools and work sites : greater than 10% absenteeism on any day, most likely due to ILI.
Residential institutions : two or more cases of ILI within a seven-day period, including at least one laboratory-confirmed case.

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