Sometimes people who are moderately to severely immunocompromised do not build enough (or any) protection when they first get a vaccination. When this happens, getting another dose of the vaccine can sometimes help them build more protection against the disease, including COVID-19. The Centers for Disease Control and Prevention (CDC) recommends moderately to severely immunocompromised people consider receiving a third dose of an mRNA COVID-19 vaccine (Pfizer-BioNTech or Moderna) at least 28 days after the second dose. The Johnson & Johnson vaccine is not included in the recommendation at this time.
In contrast, a booster dose refers to another dose of a vaccine that is given to someone who built enough protection after vaccination, but then that protection decreased over time and needs a boost.