Flu (influenza): For health professionals

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Key information

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

While most people recover in 3 to 7 days, severe illness can occur. Some groups are at a greater risk of influenza-related complications.

Getting vaccinated against influenza each fall reduces the risk of influenza and serious illness.

It is especially important for people to get the influenza vaccine to reduce:

  • the morbidity and mortality associated with influenza
  • any further pressure on the health care system during the respiratory virus season

For advice regarding the administration of seasonal influenza vaccines, refer to:

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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. hemagglutinin (HA)
  2. neuraminidase (NA)

Of these, the influenza A viruses that have caused widespread human disease have been:

  • 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, there can be subtle changes in the surface protein of influenza viruses. This is known as antigenic variation, or antigenic drift. Antigenic drift results in multiple strains within an influenza A subtype or B lineage. The ever-present possibility of antigenic drift requires seasonal influenza vaccines to be reassessed annually.

In most seasons, 1 or more of the strains in the influenza vaccines are changed from the previous season.

As well, the immune response to seasonal influenza vaccines decreases over time. This is known as waning vaccine-induced immunity.

Antigenic drift resulting in possible new formulations of the vaccine and waning vaccine-induced immunity are reasons why the influenza vaccine is recommended before each influenza season.

More information on approved vaccines for the 2023–2024 influenza season can be found in appendix B of the NACI statement on seasonal influenza vaccine for 2023–2024.

Epidemiology of influenza

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

According to data from before the COVID-19 pandemic, each year in Canada, it is estimated that influenza causes approximately:

  • 12,200 hospitalizations
  • 3,500 deaths

International burden of disease

According to data from before the COVID-19 pandemic, 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


Influenza is primarily transmitted by droplets and small particle aerosols spread through coughing, sneezing, talking and breathing. It may also be transmitted through direct or indirect contact with infected respiratory secretions on surfaces and objects.

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 symptoms start.

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

Spectrum of clinical illness

Influenza virus infection causes a wide spectrum of illness, from asymptomatic to severe. While most people recover in 3 to 7 days, severe illness can develop, which can result in hospitalization or death.

Influenza symptoms usually come on suddenly. Symptoms can include some or all of the following:

  • high fever
  • cough
  • muscle aches
  • headache
  • chills or feeling feverish
  • fatigue
  • loss of appetite
  • sore throat
  • runny or stuffy nose

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

Symptoms of influenza can be similar to symptoms of COVID-19. For more information, refer to COVID-19 signs, symptoms and severity of disease: A clinician guide.

Some groups are at greater risk of hospitalization and complications, such as pneumonia and worsening of underlying medical conditions.

Risk groups for influenza-related complications

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

  • adults 65 years of age and older
  • all children younger than 5 years of age
  • people who are pregnant
  • adults and children with the following chronic health conditions:
    • cardiac or pulmonary disorders
    • diabetes mellitus and other metabolic diseases
    • cancer and other immunocompromising conditions due to underlying disease and/or therapy
    • renal disease
    • anemia or hemoglobinopathy
    • neurologic 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)
  • people of any age who are residents of nursing homes and other chronic care facilities
  • Indigenous peoples

The National Advisory Committee on Immunization (NACI) is an external advisory body to the Public Health Agency of Canada. NACI makes recommendations on the optimal use of vaccines available in Canada.

The following NACI statement elaborates on people at high risk of influenza-related complications and for whom influenza vaccination is particularly recommended:

Prevention and control

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

Every year, NACI issues a statement on seasonal influenza vaccines. It informs immunization programs and health care providers regarding the use of these vaccines based on the most current data available.

NACI recommends an influenza vaccine for all people in Canada aged 6 months and older who do not have contraindications to the vaccine. When children 6 months to less than 9 years of age receive an influenza vaccine for the first time, they should be given 2 doses a month apart. Afterwards, children only need 1 influenza vaccine dose each fall. If the influenza vaccine is given for the first time to an individual 9 years of age or older, only 1 dose is recommended for that year.

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

  • at high risk of influenza-related complications
  • 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 5 years 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 specifically include those who:

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

Health care providers should offer an authorized trivalent (3 strains) or quadrivalent (4 strains) influenza vaccine to eligible people 6 months and older when it becomes available in the fall. Decisions about the timing of vaccination programs and vaccines available in a given jurisdiction are made by provinces and territories considering:

  • local influenza activity
  • programmatic factors

It is preferable to administer influenza vaccines before the onset of the influenza season. Delayed administration may result in lost opportunities to prevent infection from exposures that occur prior to vaccination. However, vaccination may still be offered until the end of the season.

For people 6 months of age and older, the administration of the seasonal influenza vaccine can be given:

  • at the same time as other vaccines, including a COVID-19 vaccine or
  • before or after other vaccines, including a COVID-19 vaccine

To help prevent the spread of influenza, health care providers can:

  • receive influenza vaccine themselves to help prevent transmitting influenza to their patients
  • discuss the risks and benefits of the influenza vaccine with patients, as well as the risks of not being vaccinated
  • use every opportunity to administer the influenza vaccine, even after influenza activity has been documented in the community
  • give recommended vaccines that are still needed at the same visit, including influenza, COVID-19 and pneumococcal vaccines, as required
  • inform patients about public health measures to reduce and prevent the spread of influenzaCOVID-19 and other respiratory viruses, which can include:
    • washing hands frequently
    • staying home when sick or infected
    • maintaining up-to-date vaccinations
    • wearing a well-constructed, well-fitting mask in indoor public spaces
  • inform patients that symptoms of influenza and COVID-19 are similar, and what to do if they start to develop symptoms

Further clinical guidance about vaccination, including vaccine administration advice and additional safety considerations, can be found in the Canadian Immunization Guide.


Most people with influenza will become only mildly ill and will not require hospitalization or antiviral medication.

Antiviral medications to reduce influenza morbidity and mortality are recommended for people with influenza symptoms in high-risk groups or who are severely ill. Antiviral medications are recommended for these individuals when influenza is circulating in the community.

Individuals who are recommended to receive antiviral medications and have symptoms of influenza do not need laboratory confirmation of influenza to receive the medication.

The most commonly used influenza antiviral medication for the treatment and prevention of influenza is oseltamivir, which is given orally.

Antiviral medication should be given as soon as possible and optimally within 48 hours of symptom onset. They can also be given beyond 48 hours in some circumstances. For information on the clinical management of influenza, consult the following guidance:


FluWatch surveillance

FluWatch, Canada's national influenza surveillance system, monitors the spread of influenza and influenza-like illnesses (ILI). It provides current information about circulating influenza strains and the extent to which they match the strains in the influenza vaccines. It also indicates what proportion of tested strains are resistant to antivirals.

For the latest information on influenza activity in Canada, visit FluWatch reports.

Primary care physicians, nurse practitioners and registered nurses can become FluWatch sentinel practitioners. For more information, visit FluWatch Sentinel Practitioner ILI Surveillance Program.

You may also encourage your patients to track influenza and COVID-19 by becoming a FluWatcher.

International surveillance

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

Webinar on influenza vaccination

Each year in the fall, the Public Health Agency of Canada releases an influenza webinar for health care professionals. This webinar series is aimed at updating health care providers on current influenza recommendations for the upcoming influenza season.

Access current webinar and presentation materials:

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