Why are two jabs now seemingly useless - but a third shot of the same vaccine a game changer?
Is it just because of the timescale?
In terms of protection from infection the timing matters. Circulating antibodies aren't supposed to just stay at high levels forever - they're supposed to kill an infection then fade away. The booster helps keep circulating antibodies high, because that helps us right now.
However initial results (test tube, clinical trials and early deployment) say that it doesn't just get you back where you were after dose two, it does better. You get higher circulating antibodies, but you also improve the number and variety of the immune system memory cells that respond to the covid virus.
Again that's a normal immune system response. If we see a threat keep coming back, the immune system assumes it will happen again and prepares accordingly. The immune system also has a, "and things like that," kind of response, that helps it start turning out modified antibodies when it sees a new variant.
It's those memory cells that help to protect us from severe disease even if we catch it.