It wasn't really "developed quickly". It uses the same principles and methods that have been used to produce other influenza vaccines. The only thing that's different this time is the antigens which are from swine flu influenza instead of the yearly seasonal influenzas. There's nothing particularly novel about the vaccine that needs broad-scale testing first. It's a very well characterised, tested, and safe process. Which is why it was able to be bought into use quickly.
edit: Also rest assured that in the very rare event that there are serious side-effects they'll be picked up very quickly, as each reported adverse side effect if collected and collated.