There’s a bunch of articles out there, but the primary reason new vaccines generally take years to develop isn’t because they’re doing all this extra precautionary research and studies. It’s because they lack funding.
In this case, funding wasn’t an issue which drastically speeds up the timeline. Not to mention, there was already years of existing research underway on mRNA vaccines and coronavirus. We’re talking hundreds of millions of people who have received the mRNA vaccines with very high effectiveness and very few side effects. I’d consider that pretty proven.