Well, it stimulates a very focused part of the immune system, not every part of the immune system. The immune system is made up of a whole lot of what we're made up of. It's quite complex. It includes beneficial organisms that live symbiotically within us and in order to stay alive protect us, their host. It involves nutrients we consume that help prevent oxidation, organ weakness, etc., and nutrients that directly create part of the immune system. What vaccines do is, in different ways depending on the vaccine, tell part of our immune system to be on the lookout for certain aspects of the disease being vaccinated against. In the case of covid, they all target the distinctive spike protein corona viruses use to turn our immune systems against us and let them get to where they need to in order to replicate. If you're not exposed to that very specific aspect the vaccine targets, your immune system won't do anything at all it doesn't usually do. So no, it doesn't repress the immune system, it readies it to attack one very tiny piece of the random stuff floating around our world that can make us sick. Now, it is possible that if one gets a ton of vaccines at one time it might produce so much temporary inflammation from the immediate immune response to the vaccines that is what teaches the immune system to protect us if we are again exposed to that feature that we might have too much of an immune response, which is why some advocate for spacing out childhood vaccines more than we do now, but that doesn't apply to just getting the one covid vaccine. You can also have a bad reaction to a vaccine that can overwhelm your immune system as well, but all medicine is a risk and the benefits have to greatly outweigh the risk to make the medicine appropriate to use, but again, with pandemics, that's a pretty easy answer. Peace.
It activates the immune system, which is then able to respond to the coronavirus SARS-CoV-2 and prevent it from causing the illness Covid-19.
Vaccines expose the body to a little bit of a virus or illness (in a form that won't make the person sick) and that causes the body to create antibodies in reaction. Those antibodies (created by the person's immune system) then protect against the virus or illness.
It doesn't "knock out" anything; if anything, it stimulates the immune system.