Vaccines need long periods of testing. Because unlike most drugs they are given to healthy individuals not to sick people who face risk from their disease. So the FDA is extra strict. Furthermore there is pressure to use the slow egg based method now used in flu vaccines
There is the possibility that Cvid 19 will mutate and make vaccines ineffective though I understand this is less likely with large complex viruses like corona viruses.
It will take a long time to make it available for everybody. Will they use a priority system giving it first to hospital workers and the elderly?