Why do you think a vaccine produced and certified outside of the U.S.A would be approved for use in the U.S.A prior to lets say the Moderna candidate that is cu

Why do you think a vaccine produced and certified outside of the U.S.A would be approved for use in the U.S.A prior to lets say the Moderna candidate that is currently already in phase 3 testing within the U.S.A? I think a better question might be: What will happen if the government of the U.S.A. purchase all (at least the first 100 million) doses of the effective vaccine(s).