I struggle with this at times, but you have to understand the logic of a simple mind. There is no nuanced journey to a conclusion. They simply connect two data points and lean into them.
They were told that once we have a vaccine in place, we'll be able to have more control over the disease. There's a lot of explanation for why that's the case, but all they heard in their heads was, "This will be over once a vaccine shows up."
This is why you have to be careful when messaging to large groups of people. Average minds don't flex unless they have to, so if you give them the wrong message, they will accept it as long as it sounds like a good thing. It also helps if it sounds easy and doesn't seem to require effort.
Trump (and by proxy his people) are really good at simple-minded messaging. This is why messages like "It will all just go away," and "A vaccine is coming soon" work on them. It requires no work on their part, and it allows them to do what they want presumably with no consequences. It's why calling this a "Chinese virus", which changes nothing about the virus' nature or preventing it, resonates with them. It makes them feel better about it by giving them someone to blame, which is petty but that's how it goes.
TL;DR: Simple minds seek simple solutions. A miracle shot in the arm right in time for Thanksgiving sounds great, whether or not it makes sense that it will happen.