<< Seasonal Immunization

Influenza What You Should Know

    What

  • Influenza is a respiratory infection caused primarily by influenza A and B viruses.
  • Seasonal influenza epidemics occur annually in Canada, generally in the late fall and winter months.
  • Prior to the COVID-19 pandemic, influenza occurred globally with an annual attack rate estimated at 5% to 10% in adults and 20% to 30% in children.
  • Common symptoms of influenza include fever, cough and myalgia. Most people will recover within a week to 10 days, but some people are at greater risk of complications, such as pneumonia. Influenza infection can also worsen certain chronic conditions and increase the risk of cardiovascular events.
  • There are 10 influenza vaccines authorized and available for use in Canada: 8 inactivated influenza vaccines (IIV), a recombinant influenza vaccine (RIV), and a live attenuated influenza vaccine (LAIV). Some protect against 3 strains of influenza (i.e., trivalent) and others protect against 4 strains of influenza (i.e., quadrivalent). For the 2025–2026 season, there is no longer a preference between quadrivalent and trivalent vaccines, as B/Yamagata lineage viruses have not been detected globally since 2020.
  • The influenza vaccines are safe and well-tolerated.

    Who

  • Influenza vaccination should be offered annually to anyone 6 months of age and older who does not have contraindications to the vaccine.
  • Influenza vaccination is particularly recommended for:
    • people at high risk of severe disease, influenza-related complications, or hospitalization
    • people capable of transmitting influenza to those at high risk, including healthcare workers
    • people who provide essential community services
    • people whose occupational or recreational activities increase their risk of exposure to avian influenza A viruses (e.g., H5N1), including those in direct contact with poultry, swine, dairy cattle, or wild birds

     How

  • Adults and children 9 years of age and older should receive 1 dose of influenza vaccine each year.
  • Children 6 months to less than 9 years of age who have never received influenza vaccine in a previous influenza season should be given 2 doses of influenza vaccine in the current season.
  • Children 6 months to less than 9 years of age who have been vaccinated with at least 1 dose of seasonal influenza vaccine in any previous season should receive 1 dose of influenza vaccine per season thereafter.

     Why

  • Vaccination is the most effective way to prevent influenza and its complications that can lead to severe disease, hospitalization and death.
  • Vaccination can help prevent the spread of influenza.
  • Annual vaccination is required because influenza viruses are constantly evolving and the body's immune response to influenza vaccination may not persist beyond a year. A new vaccine is designed each year to target the changes in the circulating influenza viruses.

 

Concurrent administration with other vaccines

All influenza vaccines, including LAIV, may be given at the same time (i.e., same day) as, or at any time before or after, administration of another live or non-live vaccine for those aged 6 months and older.

Refer to the Government of Canada Immunization Guide COVID-19 vaccines chapter for the latest guidance on the concurrent administration of influenza vaccine with COVID-19 vaccines.

 

Interchangeability of vaccines

If a child aged less than 9 years requires 2 doses of influenza vaccine in the same influenza season, it is preferable to use the same type of vaccine for both doses. However, if the type of vaccine used for the first dose is not available for the second dose, a different type of influenza vaccine may be provided. For more information refer to The Government of Canada Principles of vaccine interchangeability

 

Vaccination of specific populations: Groups for whom influenza vaccination is particularly important

Children 6 months to 59 months of age

Young children have a high burden of influenza-associated illness. The risk of serious infection and hospitalization is highest among the very young.

Pregnant women and pregnant individuals

The influenza vaccine should be offered at any stage of pregnancy using an age-appropriate non-live vaccine (IIV-SD, IIV-cc, or RIV). If a pregnancy spans two flu seasons, two doses may be received (one each season). Pregnant individuals are at higher risk for severe illness and hospitalization, and vaccination can protect against adverse outcomes like late pregnancy loss and stillbirth. It also protects infants for the first few months of life. Household contacts and caregivers of infants under 6 months, who are too young for the vaccine, should also be vaccinated to reduce the risk of influenza-related illness.

For information on immunization and breast feeding please refer to the Government of Canada Website Immunization in pregnancy and breastfeeding for additional information.

Older Adults

Adults 65 years of age and older are at greater risk of more severe complications from influenza, and influenza-attributed mortality rates increase with age. High-dose, adjuvanted or recombinant influenza vaccine (IIV-HD, IIV-Adj, RIV) formulations are recommended for adults 65 years of age and older as these vaccines appear to have increased vaccine efficacy or effectiveness as compared to the standard dose influenza vaccine in this age group

Residents in nursing homes or other chronic care facilities

Residents of nursing homes and other chronic care facilities often have 1 or more chronic health conditions and live in institutional environments that may facilitate the spread of influenza. For these reasons, residents who are adults 65 years of age and older may be prioritized for receipt of IIV-HD, IIV-Adj, or RIV if the supply of these vaccines is limited.

Persons with chronic health conditions

Certain chronic health conditions are associated with increased risk of influenza-related complications and hospitalization. Influenza infection in individuals with certain chronic diseases can also lead to an exacerbation of the chronic condition. For a list of chronic health conditions most at risk please refer to the Government of Canada website: Groups for whom influenza vaccination is particularly important 

Immunocompromised persons

People who are immunocompromised have an increased risk of morbidity and mortality from influenza. It is particularly important for all people 6 months of age and older who are immunocompromised to receive an influenza vaccine every year. Although influenza vaccination can induce protective antibody levels in a substantial proportion of adults and children with immune-compromising conditions, vaccine effectiveness may be lower than in healthy individuals.

Indigenous Peoples

Influenza vaccination is particularly important for Indigenous Peoples who tend to have higher rates of influenza-associated hospitalization and death. The increased risk of severe outcomes may be related to the presence of chronic health conditions or delays in accessing healthcare. Susceptibility to infection may also be increased due to living conditions that favour transmission.

Healthcare workers, care providers and other workers

Influenza vaccination is recommended for healthcare workers (HCWs) and other care providers including regular visitors, emergency response workers, those who work in continuing care or long-term care facilities or residences, those who provide home care for people at high risk, and students of related health care services. HCWs and other care providers who are potentially capable of transmitting influenza to those at high risk should receive annual vaccination with any age appropriate non-live influenza vaccine, regardless of whether the high-risk individual has been vaccinated. Vaccination of HCWs and other care providers decreases the vaccinated person's risk of illness, as well as the risk of transmission of influenza to patients at high risk of influenza-associated complications. Vaccination of HCWs and residents of nursing homes is associated with decreased risk of influenza outbreaks. Annual influenza vaccination is considered an essential component of the standard of care for all HCWs.

To minimize the disruption of services during annual influenza epidemics, all people who provide essential community services should consider annual influenza vaccination, as it can decrease work absenteeism due to respiratory illnesses.

Individuals with occupational or recreational exposure to avian influenza viruses


Individuals whose work or activities place them at risk of exposure to avian influenza A viruses (e.g., H5N1) should receive annual influenza vaccination. This includes workers in poultry, swine, and dairy cattle operations, and people who work with wild birds. While seasonal influenza vaccines do not protect against avian influenza, they may help reduce the risk of co-infection with both viruses.

 

The Epidemiology of Influenza

Disease Description and Infectious Agent

There are two main types of influenza viruses causing seasonal epidemics in humans: A and B. Influenza A viruses are classified into subtypes based on surface proteins hemagglutinin (HA) and neuraminidase (NA), with three HA subtypes (H1, H2, H3) and two NA subtypes (N1, N2) known to cause widespread disease. Influenza B viruses have two antigenically distinct lineages: B/Yamagata and B/Victoria, with B/Yamagata not seen since March 2020. Antigenic variation (drift) necessitates the annual reformulation of vaccines to match circulating strains.

Reservoir

Wild birds are the primary reservoir for most influenza A viruses. In humans, currently circulating viruses are A(H1N1), A(H3N2), and B, with humans as the reservoir.

Transmission

Influenza spreads via aerosols and droplets from coughing or sneezing and through contact with respiratory secretions. The incubation period is typically about 2 days. Adults can spread influenza from 1 day before symptoms to about 5 days after onset; children and immunocompromised individuals may be infectious longer.

Risk Factors

Those at highest risk for complications include individuals with chronic health conditions, long-term care residents, adults 65+, children under 5, pregnant women, Indigenous Peoples, and individuals whose occupational or recreational activities increase their risk of exposure to avian influenza A viruses (e.g., poultry, swine, and dairy cattle workers, and those working with wild birds).

Seasonal Patterns

In Canada, influenza activity rises in the fall, peaks in winter, and can last for several months. Multiple strains often circulate each season, with occasional cases and outbreaks outside the typical season.

Clinical Illness

Influenza can range from asymptomatic to severe, with symptoms like sudden fever, cough, myalgia, headache, chills, fatigue, and sore throat. Complications can include pneumonia, respiratory failure, cardiovascular issues, and worsening of chronic conditions. There is also an increased risk of myocardial infarction, stroke, and Guillain-Barre syndrome.

Disease Distribution

Before COVID-19, annual global influenza epidemics caused about 1 billion cases, 3-5 million severe cases, and 290,000-650,000 deaths. In Canada, influenza and pneumonia are among the top 10 causes of death, with an average of 40,000 laboratory-confirmed cases, 12,200 hospitalizations, and 3,500 deaths annually. Most cases go unreported. Current influenza activity in Canada is tracked on the FluWatch website.