COVID-19 booster shots are now available to hundreds of thousands of Connecticut residents who meet the requirements issued by federal health officials.

The booster shot, which was recommended by the CDC Thursday, helps enhance waning immunity.

It's recommended for people 65 and older, long-term care residents, and people ages 18 to 64 with underlying conditions and those with high risk of exposure. Pfizer’s vaccine is the only shot approved for this at the moment.

A third dose helps people with immunocompromised conditions reach stronger levels of immunity they aren't reaching after two doses. That can be given 28 days after the second vaccine dose and is approved for both Pfizer and Moderna.

"I've seen many immunocompromised patients that have come to see me in the office who have had antibody levels done following the vaccine and they have no antibodies, so a lot of the immunocompromised people are very concerned and worried and we really didn't have much to offer so we were very happy to hear when they released the criteria for anyone that was immunocompromised," says Dr. Zane Saul, an infectious disease doctor at Bridgeport Hospital.

Saul says the third dose and the booster are the exact same shot - just given to different groups of people.