VACCINE RESOURCE COMMITTEE

of the Edmonton Zone Medical Staff Association

View Full List of Vaccine Questions Here

Influenza Vaccine Question #3:

Why can people get influenza after they have the vaccine?

The influenza virus is constantly changing; sometimes the vaccine does not include a particular strain. Additionally, many people confuse influenza with other upper respiratory infections caused by other viruses.

It takes 14 days for the influenza vaccine to work. Therefore someone may develop a symptomatic infection before they are fully protected by the vaccine.

The effectiveness of the vaccine can vary and may depend on:

• health and age

• how well this year's vaccine matches this year's flu strains

Unlike vaccines that offer higher protection, such as vaccines for measles and polio, the influenza vaccine is only about 60 to 90 percent effective. This is because multiple strains of the virus circulate every year, and it is difficult for scientists to predict exactly which strains will be dominant. Following vaccination, it is possible to become infected with a strain that was not included in the vaccine. However, the vaccine will still be somewhat effective, and symptoms will be less severe.

There are also many other viruses that cause upper respiratory infections (such as COVID and RSV, for example) that have similar symptoms.

Sources:

Government of Canada: Flu (influenza): Get your flu vaccine (flu shot)
John Hopkins Medicine: 11 flu myths debunked