Flu (influenza): For health professionals
On this page
- What health professionals need to know about influenza
- Agent of disease
- Spectrum of clinical illness
- Risk groups for influenza-related complications
- Prevention and control
- Epidemiology of influenza
- Related information
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.
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:
- haemagglutinin (HA)
- 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:
- B/Yamagata/16/88-like viruses
- 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
- muscle aches and pains
Other common symptoms include:
- loss of appetite
- sore throat
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.
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
(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:
- proper hand washing
- keeping shared surfaces clean
- coughing and sneezing etiquette
- inform patients about what to do if they become ill with influenza
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:
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:
- Association of Medical Microbiology and Infectious Disease Canada (AMMI) website
- Canadian Critical Care Society’s guidance on managing patients with severe acute respiratory infection in an intensive care setting
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, 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.
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.
- National case definition: Laboratory-confirmed influenza
- Influenza surveillance (FluWatch)
- Pandemic preparedness
- Canadian Immunization Guide
- Immunization schedules: Provincial and territorial immunization programs
- Supply and Distribution of Flu Vaccine in Canada
- 2019-2020 Seasonal influenza vaccine (Pocket Guide)
- National Advisory Committee on Immunization (NACI): Canadian Immunization Guide Chapter on Influenza and Statement on Seasonal Influenza Vaccine for 2019-2020
- Immunization Competencies for Health Professionals
- Access to the seasonal flu vaccine in Canada : how the flu shot makes its way from the laboratory to the doctor's office
Infection prevention, control and treatment resources
- Seasonal influenza: Infection Prevention and Control Guidance for Management in Home Care Settings
- Guidance: Infection Prevention and Control Measures for Healthcare Workers in Acute Care and Long-term Care Settings
- Association of Medical Microbiology and Infectious Disease Canada
- Canadian Critical Care Society
- Hand Hygiene Practices in Healthcare Settings (pdf)
- The flu: don't pass it on! (video)
- A Parent's Guide to Vaccination
- Teens, Meet Vaccines
- Not Just for Kids: an Adult Guide to Immunization
- The benefits of hand washing
- Flu (influenza) awareness resources (posters and handouts)
- Find flu clinics and resources from across Canada
- Immunize Canada
- Canadian Paediatric Society: Caring for Kids
- Canadian Pharmacists Association
- CANImmunize app
- The College of Family Physicians of Canada
- Canadian Nurses Association: Seasonal influenza
- The Society of Obstetricians and Gynaecologists of Canada
- World Health Organization: Influenza
First Nations and Inuit resources
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