Flu (seasonal influenza): For health professionals
On this page
- Key information
- Agent of disease
- Epidemiology of influenza
- Transmission
- Spectrum of clinical illness
- Risk groups for influenza-related complications
- Prevention and control
- Treatment
- Surveillance
- Webinar on fall and winter respiratory illnesses
Key information
Seasonal influenza is a respiratory illness caused primarily by the influenza A and B viruses.
While most people recover within a week to 10 days, severe illness can occur. Certain people are at a greater risk of influenza-related complications.
Getting vaccinated against influenza each fall reduces the risk of serious illness from influenza.
It is especially important for people to get the influenza vaccine to reduce:
- the morbidity and mortality associated with influenza
- the possibility of transmitting influenza to those at high risk of severe outcomes from influenza
- pressure on the health care system during the respiratory virus season
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.
For advice regarding the administration of seasonal influenza vaccines and awareness resources for your patients, refer to:
- Statement on seasonal influenza vaccine for 2025–2026
- Influenza vaccines: Canadian Immunization Guide
- Risk groups for influenza-related complications
- Influenza awareness resources
- First Nations and Inuit seasonal influenza awareness materials
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:
- hemagglutinin (HA)
- 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:
- B/Victoria
- B/Yamagata
Over time, there can be subtle changes in the surface protein of influenza viruses. This is known as antigenic variation, or antigenic drift. It results in multiple strains within an influenza A subtype or B lineage. In most seasons, 1 or more of the circulating strains have changed from the previous year. This requires a reassessment and updating of the vaccine formulation to keep the product matched to the circulating strains.
Because of potential changes in the circulating influenza virus from year to year and waning of vaccine-induced immunity to influenza over time, annual influenza vaccination is recommended.
Antigenic shift due to a reassortment of genes can also occur. This can cause an abrupt, major change in an influenza A virus. The resulting reassortment of the influenza viruses can become an influenza pandemic.
Learn more:
- What health professionals need to know about pandemic influenza
- Canadian pandemic influenza preparedness: Planning guidance for the health sector
Recent changes in circulating influenza B lineage
The influenza B/Yamagata lineage has not been detected anywhere in the world since March 2020. Because of this, the World Health Organization has recommended removing B/Yamagata antigen as a component of inactivated and live attenuated influenza vaccines.
Due to these recent changes in epidemiology and global expert advice, NACI no longer has a preference between quadrivalent and trivalent influenza vaccine formulations.
More information on approved vaccines for the 2025–2026 influenza season can be found in:
Epidemiology of influenza
Influenza and pneumonia are among the top 10 leading causes of death in Canada.
According to Canadian data from before the COVID-19 pandemic, each year influenza causes approximately:
- 15,000 hospitalizations
- 3,500 deaths
Worldwide, it causes about:
- 1 billion infections
- 3 to 5 million cases of severe illness
- 290,000 to 650,000 deaths
Transmission
Influenza is primarily spread when a person who is infected releases infectious respiratory particles of various sizes into the air. It can also be spread through contact with contaminated 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 the day before symptoms start to approximately 5 days after symptoms start.
Children and people who are immunocompromised may be infectious for longer.
Spectrum of clinical illness
Influenza virus infection causes a wide spectrum of illness, from asymptomatic to severe. Most people recover within a week or 10 days. However, severe illness can develop, which can result in hospitalization or death.
Influenza symptoms usually include the sudden onset of:
- fever
- cough
- myalgias
Other common symptoms include:
- chills
- fatigue
- headache
- sore throat
- loss of appetite
- nasal congestion
In some people, especially children, nausea, vomiting and diarrhea may occur.
Some groups are at greater risk of hospitalization and other complications, such as:
- worsening of chronic health conditions
- heart complications
- pneumonia and respiratory failure
- death
Symptoms of influenza can be similar to symptoms of other respiratory illnesses, and may require the use of laboratory testing to confirm clinical suspicion.
Learn more:
- COVID-19 signs, symptoms and severity of disease: A clinician guide
- Respiratory syncytial virus (RSV): For health professionals
- RSV vaccines: Canadian Immunization Guide
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
- pregnant women and pregnant individuals
- 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 or therapy
- renal disease
- anemia or hemoglobinopathy
- neurologic or neurodevelopmental conditions
- people with a 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
- individuals in or from First Nations, Inuit, or Métis communities
The following NACI statement elaborates on people at high risk of influenza-related complications and for whom influenza vaccination is particularly important:
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 makes recommendations for immunization programs and health care providers regarding the use of these vaccines based on current data available. Full NACI guidance for the current influenza season is available, including choice of vaccine, dose and route of administration, schedule and contraindications.
Statement on seasonal influenza vaccines for 2025–2026
NACI recommends an influenza vaccine for all people in Canada aged 6 months and older who do not have contraindications to the vaccine.
Patients and providers should be aware that risks:
- of acquiring influenza are higher in some settings
- from influenza infection are higher in some individuals than others
The influenza vaccine should be offered as a priority to the groups for whom influenza vaccination is particularly important, which are people:
- at high risk of severe disease, influenza-related complications or hospitalization
- capable of transmitting influenza to those at high risk
- who provide essential community services (including health care workers)
- whose occupational or recreational activities increase their risk of exposure to avian influenza A viruses (e.g., H5N1)
Although seasonal influenza vaccines do not protect against avian influenza, they may lower the risk of co-infection with both viruses and possible viral reassortment leading to a human-transmissible virus with pandemic potential.
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 one 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.
It is preferable to administer influenza vaccines before the onset of the influenza season. It generally takes 2 weeks for immunity to develop after vaccination. 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 or
- at any time before or after other vaccines
Health care providers can also:
- receive influenza vaccine themselves to help prevent transmitting influenza to their patients
- discuss the benefits and possible side effects 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 the start of influenza activity in the community
- give other recommended vaccines at the same visit, including influenza, COVID-19, RSV, and pneumococcal vaccines, as required
- inform patients about maintaining up-to-date vaccinations
- encourage the use of personal protective measures to reduce the risk of getting or spreading influenza, COVID-19 and other respiratory infectious diseases, which can include:
- staying home when sick or infected
- wearing a well-constructed, well-fitting mask when appropriate
- improving indoor ventilation when possible
- cleaning hands regularly
- covering coughs and sneezes
- regularly cleaning and disinfecting high-touch surfaces and objects
Further clinical guidance about vaccination, including vaccine administration advice and additional safety considerations, can be found in the Canadian Immunization Guide.
Learn more:
- Approved vaccines: Appendix B of the NACI statement on seasonal influenza vaccine for 2025-2026
- Summary: Statement on seasonal influenza vaccines for 2025–2026
- Risk groups for influenza-related complications
- Key immunization information: Canadian Immunization Guide
- Respiratory infectious diseases: Reduce your risk with personal protective measures
- COVID-19: Prevention and risks
Treatment
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.
Individuals who have symptoms of influenza and are recommended to receive antiviral medications 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:
Surveillance
FluWatch+ surveillance
FluWatch+ is Canada's national surveillance system that monitors the spread of respiratory viruses, such as:
- RSV
- COVID-19
- flu (influenza)
- other respiratory viruses
The FluWatch+ surveillance system has an interactive dashboard. This technical dashboard provides a weekly overview of key trends on the activity of respiratory viruses.
Learn more:
International surveillance
The World Health Organization (WHO) has current international respiratory activity data.
FluWatch sentinel practitioner program for health professionals
If you're a physician, nurse practitioner or registered nurse who is involved in primary care, you can become a FluWatch sentinel practitioner.
The Sentinel Primary Care Influenza-like Illness Surveillance Program is FluWatch's primary source for outpatient flu data in Canada.
To sign up, email fluwatch-epigrippe@phac-aspc.gc.ca.
You may also encourage your patients to help track influenza and COVID-19 through the following:
Learn more:
Webinar on fall and winter respiratory illnesses
Each year, a fall and winter respiratory illnesses webinar for health care professionals is released, presented by the Public Health Agency of Canada (PHAC) in partnership with the National Collaborating Centre for Infectious Diseases (NCCID). This webinar series is aimed at updating health care providers on the prevention of respiratory illnesses during the fall and winter seasons.
Access webinar and presentation materials: