And this year, the effectiveness of influenza vaccine in the USA is around 23% or less. For effective vaccines (measles, etc) with severe consequences for infection, it makes sense, but recent research is showing that people who have previously been vaccinated for influenza are actually *more likely* to get sick with a newer strain (again, NOT an issue with the measles vaccine). For flu, I'd guess that people who are exposed to the live virus in small quantities may build more natural immunity than those that a vaccine, but research hasn't shown the cause yet. There are two ways to get immunity after all:
a) An effective vaccine
or
b) Get sick, suffer the consequences, and naturally build immunity
In the case of (b), if you're infectious before showing visible symptoms (and/or you're not willing to become a hermit until you are clear) then the vaccine is still the best route, and more in the community interest. In the US, where sick days are lacking, many people aren't willing (or able) to lose the pay either. You also end up with dipshit parents who deliberately expose their kids to nasty stuff so that they *WILL* get sick and later be immune... which just seems cruel and unnecessary.