What happens if the strains of the influenza virus included in the annual vaccine do not perfectly match the strains that are circulating and causing illness in Canada?

Influenza Immunization
"the flu shot"

• Your Flu Season Toolkit

• Who should get the flu shot?
• Find a flu shot clinic
• Wash your hands
• Practice cough etiquette
• Eat well
• Stay active
• Understanding Influenza
• Flu Prevention Checklist
• If you do get the flu
• Complications of the flu
• Related Information
• For Health Professionals
• Set up a Flu Shot Clinic at your Workplace

The influenza virus strains circulating the globe change on a regular basis. In February of each year, the World Health Organization (WHO) makes recommendation on the strains to be included in the influenza vaccine for the northern hemisphere. Two influenza "A" type viruses and one influenza "B" virus are selected based on the characteristics of the current circulating and new influenza virus strains. The selected strains are used to produce the influenza vaccine for the following fall and winter influenza season. Continual change of the circulating influenza virus means that a new influenza vaccine is reformulated each year to protect against new infections.

The effectiveness of influenza vaccination depends on several factors, including the similarity between the viral strains used in the vaccine and the strains in circulation during influenza season. With a good "match," influenza vaccination has been shown to prevent the flu in 70% to 90% of healthy individuals, and for about 50% of the elderly.

Studies show that when there is an imperfect match between the strains included in the vaccine and the circulating influenza viruses, depending on the degree of mismatch, the influenza vaccine can still reduce the overall risk of influenza infection by up to 50% in healthy adults.

A vaccine that is not perfectly matched doesn't necessarily mean that more people will get sick or that illness will be more severe. It can still offer protection against related viruses making illness milder and preventing complications.