From the CDC:
Data Supporting Need for a Booster Shot
Studies show after getting vaccinated against COVID-19, protection against the virus and the ability to prevent infection with variants may decrease over time and due to changes in variants.
Although COVID-19 vaccines remain effective in preventing severe disease, recent data pdf icon[1 MB, 68 pages] suggest their effectiveness at preventing infection or severe illness wanes over time, especially in people ages 65 years and older.
The recent emergence of the Omicron variant further emphasizes the importance of vaccination, boosters, and prevention efforts needed to protect against COVID-19.
Data from clinical trials showed that a booster shot increased the immune response in trial participants who finished a Pfizer-BioNTech or Moderna primary series 6 months earlier or who received a J&J/Janssen single-dose vaccine 2 months earlier. With an increased immune response, people
should have improved protection against getting infected with COVID-19. For Pfizer-BioNTech and J&J/Janssen, clinical trials also showed that a booster shot helped prevent severe disease.
https://www.cdc.gov/coronavirus/2019...ster-shot.html