This past years flu shot was about 45% effective. But even that’s a projection. That means if 1,000,000 got a shot, only 450,000 didn’t get the flu. The unknown is how many of those would NOT have gotten the flu without the shot.
Folks here don’t seem to realize the flu shot does not prevent the flu. It only makes it less likely but not 100%.
If you get the shot too early in the year, its effectiveness could wane, according to the Center for Disease Control and Prevention.
The flu vaccine can take the average healthy adult’s immune system up to two weeks to build up enough antibodies for protection, which means you still have a chance of getting the flu within those two weeks.
If you have a weakened or impaired immune system your chances of getting the flu are higher, even if you’ve been vaccinated.