You can actually do much worse than nothing with antibody dependent enhancement. Let's say you make something that at first glance appears similar to the virus but is a little off. Maybe you chose the wrong proteins, or maybe you used an inactivated virus and the inactivation step damaged some key part of the virus. You inject the vaccine and your body learns how to make antibodies for this particle.
Now the real virus shows up, your body goes "Aha! I've seen this before let me make those antibodies." But it wasn't trained on the real virus, so the antibodies it makes aren't actually effective against the real thing. The virus replicates and your body makes more and more antibodies but they do nothing to stop it. Meanwhile the the antibodies are running rampant in your body causing inflammation and blood clots.
This has happened before with vaccine candidates, and is analogous to what goes in on severe covid. The body tries to respond to the invading virus but does so ineffectively and causes potential deadly damage.
Isn't it possible your body would have the same response if it was exposed to a variant of a virus that it already has antibodies for?
Maybe the explanations are simplified but I don't quite understand how the immune system could fail to recognise permutations of proteins or any other differentiating factor between COVID-19, its variants and mRNA vaccines; nor what makes this specific example of vaccination riskier.
For a real-life example look into dengue fever. There are multiple variants that - to our immune system - look very similar, but antibodies for one do not protect against the others. This means that if you recover from one variant and are subsequently infected with a second you can have a really really bad time.
The reason the second infection is so bad is exactly as outlined above: your immune system 'recognises' the second variant and produces antibodies that it learnt when fighting off the first variant you recovered from. Unfortunately, these antibodies are useless against the second variant, and you have a bad time.
This exact same issue happens with the current vaccine for dengue, which is why it is only given to people who have already had dengue of some kind, or in populations where most people have had it before. For a situation where this did not happen see the section titled 'Phillipines controversy' in the wikipedia article.
There is currently a lot of work being done to produce vaccines that protect against all 4 variants at the same time, with some promise for the years ahead as trial results come in!
Your immune system is not intelligent. Through natural selection it has evolved to be able to handle some things, but it can't see permutations like that, as they are not something it evolved for, it's all chemical/physical matching.
Now the real virus shows up, your body goes "Aha! I've seen this before let me make those antibodies." But it wasn't trained on the real virus, so the antibodies it makes aren't actually effective against the real thing. The virus replicates and your body makes more and more antibodies but they do nothing to stop it. Meanwhile the the antibodies are running rampant in your body causing inflammation and blood clots.
This has happened before with vaccine candidates, and is analogous to what goes in on severe covid. The body tries to respond to the invading virus but does so ineffectively and causes potential deadly damage.