Flu (influenza): For health professionals

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What health professionals need to know

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.

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

For the 2020-21 flu season, it is especially important for Canadians to get the flu vaccine to reduce:

  • the morbidity and mortality associated with influenza
  • any further pressure on the health care system during the COVID-19 pandemic

Order the Seasonal Influenza Vaccine Pocket Guide 2020-2021 Edition.

Consult the recently developed Guidance on Influenza Vaccine Delivery in the Presence of COVID-19.

Get free flu awareness and educational products

Agent of disease

Influenza A and B are the main influenza viruses that 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
  2. B/Victoria

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. This is why a new flu vaccine is needed each flu season.

Spectrum of clinical illness

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

Influenza symptoms typically include sudden onset of:

  • high fever
  • cough
  • muscle aches and pains

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 develop. Some groups are at high risk of influenza-related complications and hospitalization.

Symptoms of the flu can be similar to symptoms of COVID-19. Refer to COVID-19 information for health professionals.


Influenza is primarily transmitted by droplets and 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:

  • pregnant people
  • children and adults with the following chronic health conditions:
    • cardiac or pulmonary disorders
    • diabetes mellitus and other metabolic diseases
    • cancer and other immune compromising conditions due to underlying disease and/or therapy
    • renal disease
    • anemia or hemoglobinopathy
    • neurological or neurodevelopmental conditions
    • morbid obesity (BMI of 40 and over)
    • 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
  • adults 65 years of age and older
  • all children younger than 60 months of age
  • people who have reduced access to health care, such as vulnerable populations like those experiencing homelessness or those with disabilities
  • people who are at an increased risk of illness because of living conditions, such as those:
    • living in shelters who experience overcrowding
    • who have limited access to facilities for personal hygiene
    • persons with disabilities who might have limited capacity to understand or perform personal hygiene practices

The following statement elaborates on persons at high risk of influenza-related complications:

National Advisory Committee on Immunization (NACI): Canadian Immunization Guide Chapter on Influenza and Statement on Seasonal Influenza Vaccine for 2021-2022

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. This provides practitioners with guidance on the vaccines authorized for that season. Further clinical guidance about vaccination, including vaccine administration advice and additional 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:
      • 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 or long-term care facility

These immunization programs should also specifically include those who:

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

Trivalent and quadrivalent influenza vaccines (conferring immunity to three and four different flu strains) are authorized for use and marketed in Canada. Not all vaccine products are available in all jurisdictions. Availability of some products may be limited. Therefore, individual provinces and territories must be consulted about the products available in their jurisdictions.

Health care providers should offer influenza vaccine when it becomes available in the fall. Decisions about the precise timing of vaccination programs in a given geographic area are made considering:

  • local influenza activity
  • programmatic factors

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

Timing, location and logistics of influenza vaccinations may be impacted by COVID-19. Refer to recent advice on the delivery of influenza vaccine in the presence of COVID-19 for guidance.

To help prevent the spread of influenza and COVID-19, health care providers can:

  • receive influenza vaccine themselves to help prevent transmitting influenza to their patients
  • use every opportunity to administer the flu vaccine to people at risk, even after influenza activity has been documented in the community
  • discuss the risks and benefits of the flu vaccine with patients, as well as the risks of not being vaccinated
  • teach patients about public health measures to reduce and prevent the spread of influenza and COVID-19
  • inform patients that symptoms of influenza and COVID-19 are similar and what to do if they start to develop symptoms


Most people with influenza will become only mildly ill and do not need medical care or antiviral medication.

Health care providers may wish to consider prescribing antiviral drugs to reduce influenza morbidity and mortality, especially for people in high-risk groups or who are severely ill.  Antivirals drugs licensed in Canada for influenza treatment and/or prophylaxis include:

  • oseltamivir
  • zanamivir
  • peramivir

Their use will depend on a number of factors, such as:

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

For information on the clinical management of influenza, consult the following guidance:

Epidemiology of influenza in Canada

Influenza and pneumonia are 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 surveillance

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, review the current FluWatch report.

You may also encourage your patients to become a FluWatcher.

International surveillance

Every year, worldwide seasonal influenza causes an estimated:

  • 1 billion infections
  • 3 to 5 million cases of severe illness
  • 290,000 to 650,000 deaths

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

Related information

Vaccine resources

Communicating with patients

Infection prevention, control and treatment resources


Educational resources

Partner resources

First Nations and Inuit resources

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