The first dose is the first exposure if you like and the second dose is a booster where the body responds again to the same threat. The second dose should give a stronger and therefore longer lasting antibody response because the body knows what to do this time but nobody really knows how long.
What's poorly understood though is the immune systems response elsewhere (T-cells etc) and this is probably why the efficacy results were so impressive, unbelievably impressive actually. A study I read has said most people may be resistant to COVID hence the large number of asymptomatic/mild cases. This is thanks to pre-existing T-cell memory and not antibodies which are non-existant in the absence of prior infection/vaccine. Vitamin D apparently is also a major indicator given the demographics of the people who get really sick as most are known to be Vitamin D deficient.
www.bmj.com
Although we can't take the chance, it's very likely that the vast majority of people will not need the vaccine because upon exposure to COVID they'll get the exact same asymptomatic/mild symptoms that they'd get had they caught COVID without a vaccine. This is perhaps why the governments accelerated programme to get the first dose out quickly is actually quite a good idea (although indeed a risk).