It depends complete on what kind of virus it is. All of the interesting research required to hit "print" on mRNA-1273 was already done in MERS. It would have be

It depends complete on what kind of virus it is. All of the interesting research required to hit "print" on mRNA-1273 was already done in MERS. It would have been surprising if the vaccine didn't work. If a new pandemic emerges from a less well understood family of viruses for which we don't have any good preclinical vaccines, it could still take decades. Sticking the genome into synthetic mRNA isn't the rate limiting step, it's knowing to target the spike protein, inserting the stabilizing proline mutations, &c and also having experience getting safe and protective responses from a similar vaccine for a related virus.