Recommendation on Repeated Seasonal Influenza Vaccination

The following highlights key information for immunization providers. Please refer to the remainder of the statement for details.

1. What

Influenza is a respiratory illness caused primarily by influenza A and B viruses. The burden of influenza varies from year to year. Prior to the COVID-19 pandemic, influenza was responsible for an estimated 12,200 hospitalizations and 3,500 deaths annually in Canada. Influenza vaccination is repeated annually due to waning immunity and the tendency of influenza viruses to frequently mutate, requiring changes in the vaccine formulation.

Some studies from different influenza seasons have suggested that receiving the seasonal influenza vaccine in one or more previous seasons may reduce the effectiveness of the vaccine against strains circulating in the current season, while other studies have not.

2. Who

This Statement applies to all individuals 6 months of age and older who are not contraindicated to receive the influenza vaccine.

3. How

The seasonal influenza vaccine should be offered to all individuals 6 months of age and older on an annual basis, regardless of whether they received a seasonal influenza vaccine in prior seasons.

4. Why

Annual influenza vaccination reduces the morbidity and mortality associated with influenza infection. Overall, the evidence shows no difference in the effectiveness of repeated influenza vaccination compared to vaccination in the current season only. Of all the seasons investigated across many studies, only during two influenza seasons was repeated vaccination across seasons associated with a reduced effectiveness against influenza A(H3N2), compared to vaccination in the current season only. Further evaluation of the effects of repeated influenza vaccination on vaccine effectiveness (VE) is needed as there is currently no predictable association that could inform vaccine program decisions from year to year. Also, repeated vaccination including the current season is consistently more effective than no vaccination in the current season.