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Reporting Obligations

Lab confirmed cases of Influenza shall be reported to the local Health Unit.

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Aetiologic Agent

Causative agents include three types of influenza virus: A, B, and C. Types A and B are of public health importance since both have been responsible for epidemics. Influenza A viruses are further divided into subtypes based on 2 viral surface glycoproteins: hemagglutinin (H) and neuraminidase (N). There are 18 different H and 11 different N sub-types. Frequent mutation of the genes encoding these surface glycoproteins results in the emergence of new strains. Influenza B viruses are comprised of two lineages, Victoria and Yamagata.

Clinical Presentation

Influenza is an acute respiratory infection (ARI). Symptoms include, but are not limited to, new or worsening cough, shortness of breath, fever, sore throat, headache, myalgia, and lethargy. Infections in children may also be associated with some gastrointestinal symptoms such as nausea, vomiting and diarrhea, while the elderly may not mount a fever response and may present with an exacerbation of underlying conditions. In most people, illness resolves within five to seven days, however the very young and adults over 64 years are at highest risk of complications such as pneumonia, exacerbation of underlying conditions, encephalitis, sinusitis, myocarditis and middle ear infections. Many individuals infected with the influenza virus are asymptomatic.

Modes of Transmission

Influenza virus particles are predominantly spread via droplets larger than five microns in diameter, which are released or shed from infected persons when they sneeze, cough, or talk. These large droplets do not stay suspended in the air and usually travel less than two metres (six feet). They may enter the host’s eyes, nose or mouth or fall onto surfaces in the immediate environment. Some of these viruses may remain viable for extended periods of time, therefore contact transmission can occur by touching contaminated objects or surfaces and then touching one’s face or eyes.

Incubation Period

Usually one to four days, with a mean of two days.

Period of Communicability

May become infectious during the 24 hours prior to onset of symptoms; viral shedding in nasal secretions usually peaks during the first 3 days of illness and ceases within 7 days but can be prolonged in young children and the immunocompromised.

Risk Factors/Susceptibility

Vaccine preventable; new vaccine is required annually the components of which depend on circulating strains. Immunity is generally achieved within 2 weeks following immunization. Risk factors include: chronic illness/underlying medical condition, immunocompromised, travel, and not immunized for influenza.

Diagnosis and Laboratory Testing

A confirmed case: clinically compatible signs and symptoms with laboratory confirmation or an epidemiologic link to a laboratory-confirmed case.

Lab Testing: The specimen of choice for seasonal influenza virus is the nasopharyngeal swab (NPS) taken within the first four days of illness. When indicated and possible, lower respiratory tract specimens (e.g. bronchoalveolar lavage) should also be submitted, as these may have greater sensitivity than NPSs.

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Treatment and Case Management

The best prevention measure is annual immunization. All Ontario residents aged 6 months and older are eligible to receive publicly funded influenza vaccine yearly.

Treatment is under the direction of the attending Health Care Provider. Advise the individual to stay away from work and school when ill and limit exposure to others, especially those at high risk for complications.

People at high risk of Influenza-related complications or hospitalization include:

  • All children 6–59 months of age
  • Adults (including pregnant women) and children with chronic health conditions including morbid obesity
  • People of any age who are residents of nursing homes and other chronic care facilities
  • People ≥ 65 years of age
  • Pregnant women
  • Indigenous peoples

Patient Information

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Additional Resources

The Flu |

Ontario Hospital Association. “Influenza Surveillance Protocol for Ontario Hospitals.”

Public Health Agency of Canada. “National Advisory Committee on Immunization (NACI), Statement on Seasonal Influenza Vaccine. “

Health Care Provider Qs & As: Influenza Immunization Information for individuals 6 months to 64 years of age

Health Care Provider Qs &As: Influenza Immunization Information for individuals > 65 years of age

Antiviral Medications for the 2023-24 Seasonal Influenza: Public Health Considerations

Heymann, D.L. Control of Communicable Disease Manual (19th Ed.). Washington, American Public Health Association, 2008.


Ministry of Health and Long-Term Care, Infectious Diseases Protocol, 2022.