Only certain populations initially vaccinated with the Pfizer-BioNTech
vaccine can get a booster shot at this time.

Older adults and 50-64 year old people with medical conditions people aged 65 years and older and adults 50–64 years with underlying medical conditions should get a booster shot of Pfizer-BioNTech vaccine. The risk of severe illness from COVID-19 increases with age, and can also increase for adults of any age with underlying medical conditions.

Long-term care setting residents aged 18 years and older.

Residents aged 18 years and older of long-term care settings should get a booster shot of Pfizer-BioNTech vaccine. Because residents in long-term care settings live closely together in group settings and are often older adults with underlying medical conditions, they are at increased risk of infection and severe illness from COVID-19.

People with medical conditions aged 18-49 years.

People aged 18–49 years with underlying medical conditions may get a booster shot of Pfizer-BioNTech vaccine based on their individual benefits and risks. Adults aged 18–49 years who have underlying medical conditions are at increased risk for severe illness from COVID-19. However, that risk is likely not as high as it would be for adults aged 50 years and older who have underlying medical conditions. People aged 18–49 years who have underlying medical conditions may get a booster shot after considering their individual risks and benefits. This recommendation may change in the future as more data become available.

Employees and residents at increased risk for COVID-19 exposure and transmission.

People aged 18–64 years at increased risk for COVID-19 exposure and transmission because of occupational or institutional setting may get a booster shot of Pfizer-BioNTech vaccine based on their individual benefits and risks. Adults aged 18–64 years who work or reside in certain settings (e.g., health care, schools, correctional facilities, homeless shelters) may be at increased risk of being exposed to COVID-19, which could be spreading where they work or reside. Since that risk can vary across settings and based on how much COVID-19 is spreading in a community, people aged 18–64 years who are at increased risk for COVID-19 exposure and transmission because of occupational or institutional setting may get a booster shot after considering their individual risks and benefits. This recommendation may change in the future as more data become available.

Examples of workers who may get Pfizer-BioNTech
booster¹ shots.

• First responders (e.g., healthcare workers, firefighters, police,
  congregate care staff)
• Education staff (e.g., teachers, support staff, daycare workers)
• Food and agriculture workers
• Manufacturing workers
• Corrections workers
• U.S. Postal Service workers
• Public transit workers
• Grocery store workers

Additional details here:
Booster Shot CDC