For health professionals: Flu (influenza)

What health professionals need to know about influenza

Influenza is a respiratory illness caused primarily by the influenza A and B viruses.

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

It is estimated that 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

Getting vaccinated against influenza each autumn is the best way to help prevent influenza infection.

There are antiviral drugs, including oseltamivir, zanamivir and peramivir currently authorized for influenza treatment and/or prophylaxis in Canada.

Agent of disease

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

Influenza A viruses are classified into subtypes based on 2 surface proteins:

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

Of these, the influenza A viruses that have caused widespread human disease over the decades are:

  • 3 subtypes of HA (H1, H2 and H3)
  • 2 subtypes of NA (N1 and N2)

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. The ever-present possibility of antigenic drift requires seasonal influenza vaccines to be reformulated annually. Antigenic drift may occur in one or more influenza virus strains.

Spectrum of clinical illness

Influenza virus infection causes a wide spectrum of illness. It ranges from asymptomatic to mild, uncomplicated illness to severe, complicated illness.

Influenza 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.


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 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 groups for influenza-related complications

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

  • all pregnant women (risk increases with length of gestation)
  • adults and children with the following chronic health conditions:
    • cardiac or pulmonary disorders
    • diabetes mellitus and other metabolic diseases
    • cancer and other immune compromising conditions
    • renal disease
    • anemia or hemoglobinopathy
    • neurologic or neurodevelopment conditions
    • morbid obesity (BMI greater than or equal to 40)
    • children up to 18 years of age undergoing treatment for long periods with acetylsalicylic acid (ASA)
  • residents of nursing homes and other chronic care facilities
  • people 65 years of age and older
  • all children younger than 60 months of age
  • Indigenous 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 2017-2018. The statement elaborates on persons at high risk of influenza-related complications.

Prevention and control

Annual influenza vaccination is the most effective way to help 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 regarding influenza vaccination, including vaccine administration advice and safety considerations, can be found in the Canadian Immunization Guide.

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

To reduce the morbidity and mortality associated with influenza, immunization programs should focus on people 6 months of age and older who are:

  • at high risk of influenza-related complications
  • 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, including:
      • household contacts of infants less than 6 months old
      • members of a household expecting a newborn during influenza season
    • those providing regular child care to children younger than 60 months of age
    • those providing services in closed or relatively closed settings to those at high risk, such as a ship's crew

These immunization programs should also include other people, specifically those who:

  • provide essential community services
  • are directly involved in culling operations with poultry infected with avian influenza

Trivalent and quadrivalent influenza vaccines are 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 offer influenza vaccine when it becomes available in the fall. Decisions regarding the precise timing of vaccination programs in a given geographic area would take into consideration:

  • 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.

To help prevent the spread of influenza, it is recommended that health care providers:

  • receive influenza vaccine themselves to help prevent transmitting influenza to their patients
  • 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
  • teach patients about influenza prevention practices, including:
    • proper hand washing
    • coughing and sneezing etiquette
    • keeping shared surfaces clean
    • what to do if they become ill with influenza


Most people with influenza become only mildly ill and do not need medical care or antiviral medication. 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 antiviral medication to reduce influenza morbidity and mortality.

There are antivirals drugs, including oseltamivir, zanamivir and peramivir, authorized for influenza treatment and/or 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:

Epidemiology of influenza in Canada

Influenza is ranked among the top 10 leading causes of death in Canada.

Each year in Canada, it is estimated that influenza causes approximately:

  • 12,200 hospitalizations
  • 3,500 deaths

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.

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

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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