For health professionals: Flu (influenza)

Get detailed information on influenza for health professionals.

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What health professionals need to know about flu

Influenza is a highly contagious respiratory infection.

It is caused primarily by the influenza A and B viruses.

Seasonal influenza causes approximately 12,200 hospitalizations and 3,500 deaths in Canada each year.

FluWatch, Canada's national influenza surveillance system, provides up-to-date information about currently circulating influenza strains

While most people recover in 7 to 10 days, severe illness can occur. Some groups are at a greater risk of influenza-related complications.

Influenza can be prevented by an annual seasonal influenza vaccination.

There are antiviral drugs, including oseltamivir (Tamiflu®) and zanamivir (Relenza®), currently authorized for influenza treatment and prophylaxis in Canada.

Agent of disease

Influenza A viruses are classified into subtypes on the basis of 2 surface proteins:

  1. haemagglutinin (HA)
  2. neuraminidase (NA)

Influenza B has evolved into 2 lineages:

  1. B/Yamagata/16/88-like viruses
  2. B/Victoria/2/87-like viruses

Over time, antigenic variation (antigenic drift) of strains occurs within an influenza A subtype or B lineage. Antigenic drift, which may occur in influenza virus strains, usually requires seasonal influenza vaccines to be reformulated annually.

While there are 3 types of influenza virus (A, B and C), only influenza A and B cause seasonal outbreaks in humans.

Spectrum of clinical illness

The spectrum of illness caused by influenza virus infection has a wide range. It ranges from asymptomatic to mild, uncomplicated illness to severe, complicated illness.

Flu symptoms typically include the sudden onset of:

  • high fever
  • cough
  • muscle aches

Other common symptoms include:

  • headache
  • chills
  • fatigue
  • loss of appetite
  • sore throat
  • coryza

In some people, especially children, nausea, vomiting and diarrhea may occur.

While most people recover in 7 to 10 days, severe illness can occur. Some groups are at high risk of influenza-related complications and hospitalization.


Seasonal influenza is primarily transmitted by droplets spread through coughing or sneezing. It may also be transmitted through direct or indirect contact with infected respiratory secretions.

The incubation period of seasonal influenza is usually 2 days, but can range from 1 to 4 days. Adults may be able to spread influenza to others from 1 day before symptoms start to approximately 5 days after.

Children and people with weakened immune systems may be infectious for longer.

Risk factors

The people at high risk of influenza-related complications or hospitalization include:

  • adults, pregnant women and children with underlying health conditions, such as:
    • cancer
    • diabetes
    • cardiac disorders
    • pulmonary disorders
    • morbid obesity (BMI ≥40)
  • residents of nursing homes and other chronic care facilities
  • people aged 65 and older
  • all children 59 months of age and younger
  • all pregnant women (risk increases with length of gestation)
  • Aboriginal peoples

For more information, refer to the National Advisory Committee on Immunization (NACI): Canadian Immunization Guide Chapter on Influenza and Statement on Seasonal Influenza Vaccine for 2015-2016. The statement elaborates on persons at high risk of influenza-related complications.

Prevention and control

Annual seasonal influenza vaccination is the most effective way to prevent influenza and its complications.

Every year, NACI issues a seasonal influenza statement that informs practitioners about the vaccines authorized for use that season. Further clinical guidance, including vaccine administration advice and safety considerations, can be found in the Canadian Immunization Guide.

NACI encourages influenza vaccine for all Canadians aged 6 months and older who do not have a contraindication to the vaccine.

NACI also identifies the groups who should be particularly encouraged to receive influenza vaccination each autumn as those who:

  • are at high risk of influenza-related complications
  • are especially capable of transmitting influenza to those at high risk, including:
    • health care and other care providers in facilities and community settings
    • household contacts (adults and children) of individuals at high risk of influenza-related complications, such as:
      • household contacts of the high-risk individuals previously listed
      • household contacts of infants less than 6 months old
      • members of a household expecting a newborn during flu season
    • those providing regular child care to children 59 months of age and younger
    • those providing services in closed or relatively closed settings to those at high risk, such as a ship's crew
  • provide essential community services
  • are directly involved in culling operations with poultry infected with avian influenza

There are currently both trivalent and quadrivalent influenza vaccines authorized for use and marketed in Canada. Not all vaccine products are available in all jurisdictions and availability of some products may be limited. Therefore, individual provinces and territories must be consulted regarding the products available in individual jurisdictions.

Health care providers should decide the timing of vaccination programs in their geographic area based on factors such as:

  • local influenza activity
  • programmatic considerations

It is preferable to administer vaccines before the onset of flu season. However, vaccination may still be offered until the end of the season.

Health care providers should:

  • use every opportunity to vaccinate people at risk, even after influenza activity has been documented in the community
  • discuss the risks and benefits of the vaccine with patients, as well as the risks of not being immunized

Health care providers play an important role in preventing the spread of influenza for the following reasons.

  • When health care providers recommend a flu vaccine, patients are more likely to get it. Research shows that 8 out of 10 people take their physician's advice and get immunized.
  • When health care providers get an influenza vaccine, they reduce the risk of passing on the virus to patients. This is especially important to vulnerable people who are at higher risk of influenza-related complications.
  • Health care providers have an opportunity to teach patients about prevention practices. These include:
    • proper hand washing
    • coughing and sneezing etiquette
    • what to do if they become ill with influenza


Most people with influenza become only mildly ill and do not need medical care or antivirals. Generally, the advice is to:

  • stay at home
  • get lots of rest
  • treat the symptoms
  • avoid contact with other people (except to get medical care)

For people in high-risk groups or who are severely ill, health care providers may prescribe antivirals to reduce influenza morbidity and mortality.

There are antivirals drugs, including oseltamivir (Tamiflu®) and zanamivir (Relenza®), currently authorized for influenza treatment and prophylaxis in Canada. Their use will depend on a number of factors, such as:

  • patient risk
  • relevant history
  • duration and severity of symptoms

For more information on the use of antiviral drugs for influenza, health care providers can consult guidance provided on the:

FluWatch, Canada's national influenza surveillance system, provides up-to-date information about currently circulating influenza strains. It also indicates what proportion of those are resistant to antivirals.

Epidemiology of influenza in Canada

Each year, an estimated 10% to 20% of the population becomes infected with influenza, causing an estimated:

  • 12,200 hospitalizations
  • 3,500 deaths

For the latest information on influenza activity in Canada, view the current FluWatch report on the influenza surveillance page.

Disease distribution (global)

Every year, worldwide epidemics cause an estimated:

  • 1 billion cases of influenza
  • 3 to 5 million cases of severe illness
  • about 250,000 to 500,000 deaths

For current international activity, visit the World Health Organization's FluNet website.

For more information

Vaccine resources

Infection prevention, control and treatment resources

Educational resources

Partner resources

First Nations and Inuit resources

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