Vaccine effectiveness
Vaccine effectiveness is how well a vaccine protects you from getting sick after you have been immunized. Being immunized means you have had a vaccine to protect you against a disease.
No vaccine is 100% effective. People respond differently to diseases and vaccines because of things like age, sex at birth, pregnancy, and health conditions.
Because of these differences, it is possible to get sick even if you are fully immunized. This is called a breakthrough infection. If you are immunized and you do get sick, your symptoms will likely be milder.
For example, if you are exposed to measles and are not immunized, you have a greater than 90% chance of getting measles. The vaccine is almost 100% effective after 2 doses, so it lowers your risk almost 100% of the time.

In the example above, after being exposed to measles, 90 people out of 100 who were not immunized got sick. Out of 100 people who were immunized, 1 got sick which means the vaccine prevented 89 out of 90 cases of disease. This means the vaccine lowered the risk of getting disease by 99%.
When more people are exposed to a disease, more people get sick, even those who are immunized. This means there are more cases of a disease. It does not mean the vaccine is less effective or does not work.
Before a vaccine is approved for use in Canada, it is tested in clinical trials. Clinical trials measure vaccine efficacy, which is how well a vaccine works in a study group.
Once a vaccine is approved for use and available to the public, vaccine experts continue to monitor cases of the disease, how many people get sick, and if those people are immunized. This tells us how effective a vaccine is in the real world.