What you are looking for is affinity maturation. On first exposure to the antigen the immune system will make low affinity antibodies i.e. simple ones that are

What you are looking for is affinity maturation. On first exposure to the antigen the immune system will make low affinity antibodies i.e. simple ones that are fast to make but only stick to very specific part of the antigen. On subsequent exposures immune system will make more antibodies for different parts of the antigen so there is higher likelihood that they stick to a little altered antigen. Denmark negative effectiveness is most likely explainable by few things. Delta (even that 10%) and omicron are spreading is slightly different populations (delta more in unvaccinated, omicron more in vaccinated), behavioral differences - while unvaccinated population can access everything, testing is a hassle and there might be some testing fatigue, Yule Simpson effect - as a whole there might be negative efficiency but if you look by subcomponents (for example by age), this will disappear. The spike protein in vaccine is fixed in place by a clever RNA hack but the real one is oscillating randomly between closed and open position. In closed position (as in vaccines) it does not stick to the ACE2 receptor but it does in open position. This means that spike protein in vaccines is less bioactive and also antibodies developed against is are less dangerous because there is a immune mechanism that starts creating antibodies against antibodies (to get rid of the eventually - your body is not from rubber and can hold only limited amount of stuff and you get constantly different kind of stuff your immune system must fight off so there is not room for everything). Now the antibodies that for against antibodies that stick to the open spike protein can also stick to the ACE2 receptor i.e. your own cells and they are called autoantibodies what is kind of bad because they mark your own cells for destruction unnecessarily. So the spike protein in vaccines is by design less dangerous but it does not necessarily mean that could not cause issues. This must be researched and verified. My current pet theory is that the lipid shell is to blame for adverse effects because it is inflammatory. Being inflammatory is not bad by itself because it attracts immune reaction and makes production of antibodies more likely but it might cause problems in wrong places like in heart. It has been suspected that vaccines entering the venous blood stream because of faulty injection technique could be the cause but it has not been verified in humans. I hope that you will see this reply.