This is factually incorrect. Of the four known COVID strains in broad circulation, the Moderna and Pfizer vaccines have displayed efficacy against all of them.
The Delta variant is more contagious, but has not demonstrated vaccine escape behavior against Pfizer and Moderna [1], and although efficacy was shown to be reduced by mutations in trials, both vaccines still successfully result in complete neutralization in of the virus by antibodies in test titres (in brief: you vaccinate people, then draw their blood and expose the antibodies to virus in vitro and see whether neutralization is effective - it was).
The Delta variant is more contagious, but has not demonstrated vaccine escape behavior against Pfizer and Moderna [1], and although efficacy was shown to be reduced by mutations in trials, both vaccines still successfully result in complete neutralization in of the virus by antibodies in test titres (in brief: you vaccinate people, then draw their blood and expose the antibodies to virus in vitro and see whether neutralization is effective - it was).
[1] https://www.theguardian.com/world/2021/jun/15/the-covid-delt...