Yep, and the virus has changed a lot since those trials were conducted, and even been subjected to certain evolutionary pressures by mass deployment of the vaccines themselves, which is not something it is possible to study in clinical trials. It's also not really possible to test for OAS because trial participants (at least, those in a highly abbreviated trial like those which were conducted for the COVID vaccines) only encounter one strain of the virus.
Yes, it's happened quite often unfortunately. And not just years later. Sometimes immediately. Flu vaccine rollouts have shown this effect in the past, as have (failed) HIV vaccines. People who took the Canadian flu vaccine got Swine Flu at a higher rate for example.
The exact cause isn't known because anything that could be perceived as "anti-vaxx" doesn't get funded or honestly researched, even though it's important. But the best bet seems to be a form of imprinting. The immune system prefers to make antibodies that it's made before and does so even if it's exposed to a pathogen different enough to have evolved past those antibodies. For real epidemics this probably doesn't matter much, because they move in waves through the population, but if you repeatedly vaccinate the whole world then you're causing fixation on a long since extinct form of the spike protein. When a mutated virus turns up the body manufactures useless antibodies that don't work rather than trying to develop new ones.
It's actually not even clear that the trials showed positive effectiveness for COVID. They reported it but the way they defined "vaccinated" creates a denominator problem. Their methodology would create the appearance of initially high effectiveness that steadily decays to nothing for anything, including water. Regulators not only didn't notice this but public health agencies adopted the same methodology.
What becomes clear the moment you start learning about this stuff is, we never hear about these problems because any bad news about vaccines is immediately squashed and anyone who points it out gets banned and viciously attacked.
Sure, but that's something that trials test for before a vaccine gets to the general public.
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