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Influenza (Flu) Vaccine

About the Disease

Influenza is a respiratory infection caused primarily by influenza A and B viruses.  In Canada, influenza generally occurs each year in the late fall and winter months.  Symptoms typically include the sudden onset of high fever, chills, sore throat, cough and myalgia.  Other common symptoms include headache, loss of appetite, fatigue, and coryza.  Nausea, vomiting and diarrhea may also occur, especially in children.  Most people will recover within a week or 10 days, but some are at greater risk of more severe complications, such as pneumonia.

About the Vaccine

For a complete list of the different types of influenza vaccine available in Canada, see the National Advisory Committee on Immunization (NACI) statement on seasonal influenza vaccine

Both inactivated and live attenuated influenza vaccines are authorized for use in Canada; some are trivalent formulations (protecting against three strains of the virus) and some are quadrivalent formulations (protecting against four strains). Influenza vaccine is safe and well-tolerated.  Influenza vaccine cannot cause influenza illness because the inactivated vaccines do not contain live virus and the viruses in the live attenuated vaccine are weakened.  For more information regarding guidelines for use and contraindications against receipt of influenza vaccine, see Section VII of the BC Immunization Manual.

Inactivated influenza vaccine: The inactivated influenza vaccine is made of killed influenza viruses and is approved for those 6 months of age and older.  The inactivated influenza vaccine is administered by intramuscular (IM) injection.  For more information on the intended use of influenza vaccines by age group, see Section VII of the BC Immunization Manual.

Live attenuated influenza vaccine: The live attenuated influenza vaccine (LAIV) is made from weakened influenza viruses.  It is given as an intranasal spray into both nostrils.  It is approved for use in individuals 2 to 59 years of age and, in BC, is publicly funded for children and youth 2 to 17 years of age inclusive. The current evidence does not support a recommendation for preferential use of LAIV in children 2 to 17 years of age.  NACI continues to recommend that a quadrivalent vaccine be used in children 2-17 years of age.  If a quadrivalent vaccine is not available, trivalent inactivated influenza vaccine (TIIV) should be used. For more information on the intended use of influenza vaccines by age group, see Section VII of the BC Immunization Manual.

Who should receive the influenza vaccine?

NACI recommends that everyone 6 months of age and older receives the influenza vaccine, with rare exception.  However, it is especially important that certain individuals are vaccinated, including those who are at high risk of serious illness from the flu (such as young children, pregnant women, the elderly and people with certain medical conditions) and those able to transmit or spread influenza to those at high risk.  For a complete list of those for whom influenza vaccine is recommended and provided free in BC, see Section VII of the BC Immunization Manual, beginning page 31.

Vaccine Schedule

The influenza vaccine is usually given as 1 dose.  Children under 9 years of age who have never received the seasonal influenza vaccine require 2 doses of the vaccine, given 4 weeks apart.  The second dose of seasonal influenza vaccine improves the immune response, as children in this age group are less likely to have been exposed to an influenza virus in the past.  Children under 9 years of age who have previously received one or more doses of seasonal influenza vaccine should receive one dose of influenza vaccine in subsequent seasons.  For further information, see Section VII of the BC Immunization Manual.

Vaccine Effectiveness

The seasonal influenza vaccine is the best way to protect against influenza.  How well the vaccine works varies from season to season.  It depends on how well matched the influenza vaccine is with the viruses circulating during the season and on the health and age status of the person being vaccinated.  When the influenza vaccine is well matched with the circulating viruses, it prevents influenza in about 60% to 80% of adults and children receiving the vacccine. Effectiveness is less in the elderly population but studies have demonstrated that the influenza vaccine decreases the incidence of pneumonia, hospital admissions and death in the elderly population. For further infomation on the seasonal influenza vaccine effectiveness and immunogenicity, see the NACI statement on seasonal influenza vaccine

More Information on Influenza

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