How do boosters work?
The heavily-mutated and contagious Omicron variant has reduced the ability of two doses of the vaccine to protect us from catching Covid. So, why do boosters work while two doses struggle?
Vaccines educate your immune system to fight coronavirus. The first dose is like primary school and teaches it the basics. Your second is secondary school building on this knowledge. The third is like university - dramatically deepening your immune system's understanding. The protection you're left with after the third dose is bigger, broader, and more memorable than you had before.
Immunity against a virus is almost never absolute - you can get re-infected, but you want a re-infection so trivial you don't know you've got it or it's mild.