Currently, when we speak about third doses, we’re speaking about an extra dose apart from the first two doses of the messenger RNA vaccine because the first two doses didn’t give sufficient protection against COVID-19. And that is why some people need a third dose for them to get to a level of immunity that’s protective.
https://health.clevelandclinic.org/covid-booster-shot/
https://health.clevelandclinic.org/covid-booster-shot/