An additional primary dose is given to people with moderately to severely compromised immune systems. The additional primary dose of an mRNA COVID-19 vaccine is intended to improve immunocompromised people’s response to their vaccine primary series.
A booster shot is given when a person has completed their vaccine primary series (which is 3 doses of the vaccine if you are immunocompromised) to enhance or restore protection against COVID-19 which may have decreased over time.