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.
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.
The CDC recommends that people stay up to date with their vaccinations and get boosted when eligible. People who are moderately or severely immunocompromised have different recommendations for COVID-19 vaccines. Use CDC’s COVID-19 vaccination schedule, or answer a few questions on the CDC website to make sure you are up to date.