As COVID-19 has shown the strain variations, various mutants, if we developed vaccination at one place, r we sure that perticular gene has not mutated take example of E-gene of coronavirus.
The possible way to find out the common antigen in corona virus is through the laboratory expert like the genetic biochemist to fine the common and differences of each newly discovered strain of COVID-10.
It will include both in silico and wet lab approaches to find region that is not mutated and is same in all populations of the world. This in my opinion is very unlikely. So one vaccine for all the populations in the world might not be a workable solution but lets hope for the best.