Boosters presently are based on vaccines developed to combat the original virus, not agsinst evolving kinds that are resistant to it, like the widespread dominant Delta variant.
And now the Omicron variant.
The omicron variant is more transmissionable and avoids antibodies more effectively, and after less than a month since it has been detected, is already in Hong Kong, Australia, Belgium, Germany and Israel, as well as the widely-reported 8 African countries.
Will there be a variant-combating booster shot? Or just pump more of the same from the original batch.
It would be hard to say one person is fully vaccinated and another is not based on the fact that one got a booster that clearly doesn't stop the new variants!