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It reduces the risk of it developing into a full infection, and it reduces the severity of an infection that does develop

Infective diseases aren't binary things, there's a gradient of how badly it hits you. That's why catching the same illness at different times might hit you with varying levels. You can think back to the three times you had COVID and how bad each instance of it was

The "battle" analogy works well for most of the infective diseases stuff, and different diseases behave in different ways.

Why do some vaccines work better

• Vaccines tell you what the infective agent looks/behaves like. It might be a part of the infective agent, a killed one, a weakened one, or it might be some byproduct it creates, etc. Depending on how good that message is, the vaccine will work better or worse.

  • What method to use also depends on what the infective agent is. If it's very similar to something harmless, you don't want to be triggering false positives. Lots of immune diseases are caused by the immune system becoming active when it shouldn't. So sometimes you CAN'T (with our current knowledge) make it any stronger

• Some diseases, like polio, need time to settle, and they need to go through certain steps (think setting up camp) before they cause the visible symptoms. Part of that is also dependent on where the infective agent enters, and where it can cause the harm/symptoms (nervous system vs respiratory tract). This is also where the vaccine is important. Maybe you still get the cold symptoms, but the vaccine makes it so COVID doesn't get to the point where it messes up your sense of smell.

• Another aspect of vaccines is to stop transmission. Similar to the reasons above, how well it can stop transmission will vary. Also important is how many people get the vaccine and how fast it is spreading. Diseases that spread rapidly, or cases when not enough people are vaccinated, will make it hard to eradicate

Some other ways different infections might be better/worse

• it depends on how badly you were exposed. If a whole lot of viruses manage to land and make camp in/on a part of the body where they can multiply, your infection will be worse.

• it depends on how strong your immune system was / how well it was functioning. This is pretty self explanatory, and depends on things like diet, rest, stress, etc.

A fun sub point to the above is that maybe you got exposed a bunch of times, where a tiny landing party arrived and got killed off easily. This might be part of why some people never get a bad infection without the vaccine (in addition to being lucky in general). Maybe the person had a bunch of near misses and built up immunity that way.

Why get COVID vaccine

Like people said elsewhere, it lowers risk a significant amount.

Yes there is a risk of certain complications, but those complications are also present (in a much worse severity) with the disease itself. So if you're at risk from the vaccine, you're likely at a bigger risk from the disease and you should talk to your doctor to weigh the pros and cons (do you take the vaccine and have them monitor you for a bit, or do you isolate entirely and mask up more than others).

Ultimately it's hard to prove that the vaccine helped you without having two timelines and comparing what happened to you in each. But with our current understanding of immunology and microbiology, the vaccine most likely helped you