An effective vaccine is particularly hard to develop because the ‘spike’ evolved to fit the hACE2 receptor on our epithelial cells is a near-perfect fit (a lot of bio processes work around how well molecules ‘fit’ together — like 3D puzzles, if you will). The fit is nearly as good as our own antibodies, so we’d need lots of them to counter the virus.

The good news in this is that any mutation of the virus is likely going to make that ‘fit’ less good. so it’s unlikely to become more deadly.

I’d also guess (as someone who’s not a medical pro) that this good fit might have something to do with the immune system over-reaction seen in some unfortunate patients.