It is understandable that individuals often confuse or interchange the meanings for a “booster” and “additional dose” of COVID-19 vaccine. According to the CDC, a booster shot is administered when a person has completed their vaccine series and protection against the virus has decreased over time. Additional doses are administered to people with moderately to severely compromised immune systems. This additional dose of an mRNA-COVID-19 vaccine is intended to improve immunocompromised people’s response to their initial vaccine series. See more about this topic from the CDC here.