These are approximated numbers I was given by a health worker here, they may vary from vaccine to vaccine and person to person.
1- The vaccine do not creates complete immunization till 28 days after the second doze application.
2- The first doze you have develop some inmunity after 21 days. But this is not complete immunization.
3- That is why the second doze is put 21 days after. 28 days after the second doze you should have a chance of 95% of not getting infected.
4- So its almost 50 days after the first vaccine till the body develop antibodies. We are not computers, is not like updating the virus data base is instant.
5- After those 50 days you only have a 5% of actually getting covid and unless really unlucky or have other medical complications you will not die from it even if you get it.
6- Those 50 days are crucial and you should take extra care, your immune system is already engaged dealing with the vaccine, do not add the real virus to the mix.
7- Do not get overconfident after the first doze. This has lead to a awful lot of people actually getting covid after the first doze. It is a shame to lose all those first shots.
8- This is why you should follow the safety measures before, during and after vaccination.
So, that small amount of people that ate vaccinated but can still get covid can spread it around. They dont die from it and even could not present symptoms but are potential vectors to other unvaccinated people.
That is why. And that is why if you are tired of masks, as 99% of the world population is, keep them only a few months more and we'll be done with this shit for good until the next one.