Last Thursday, October 21st, the CDC's Advisory Committee on Immunization Practices updated their recommendations for COVID-19 Booster doses in certain populations.
For those who received a Pfizer-BioNTech or Moderna COVID-19 series, the below groups are recommended for a booster shot 6 months or more after their initial series:
65 years and older
Age 18+ who live in long-term care settings
Age 18+ who have underlying medical conditions
Age 18+ who work or live in high-risk settings
For those who received the Johnson & Johnson COVID-19 vaccine, booster shots are recommended for those who are 18+ and were vaccinated at least two months ago.
CDC's Media Statement: https://www.cdc.gov/media/releases/2021/p1021-covid-booster.html