The virus mutates all of the time. Each time it enters a cell and then makes copies of itself. Sofar the consensus seem to be the mRNA vaccines should proof effective.
Most mutations aren't viable. Given the low fidelity of RNA replication (at least compared to DNA replication) it's remarkable just how close each copy is in a large population. Only a few mutations prove to be viable. Much less prove to be strong enough to overpower the current population.