Flu (influenza): For health professionals

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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.

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

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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. 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 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.

Transmission

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:

  • 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

Related information:

National Advisory Committee on Immunization (NACI): Canadian Immunization Guide Chapter on Influenza and Statement on Seasonal Influenza Vaccine for 2018-2019

(This 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. 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:
    • healthcare 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

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 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.

Healthcare 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.

To help prevent the spread of influenza, healthcare providers can:

  • 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 vaccinated
  • teach patients about influenza prevention practices, including:
  • inform patients about what to do if they become ill with influenza

Treatment

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

Healthcare 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 guidance on the clinical management of influenza, healthcare providers can consult the:

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

Surveillance

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.

International surveillance

Every year, worldwide seasonal epidemics cause an estimated:

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

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

Related information

Vaccine resources

Publications

Infection prevention, control and treatment resources

Publications

Educational resources

Partner resources

First Nations and Inuit resources

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