Additional dose: When someone with a weakened immune system gets an additional dose of vaccine, it is not a booster. It is part of the first series of shots needed for the person to be fully vaccinated. Many people with impaired immunity did not have a full response to their first doses. Booster shot: An additional dose is a booster when someone had a full response to vaccination, but effectiveness is expected to wane over time. The booster would re-energize their immune response.