U.S. Centers for Disease Control and Prevention director Dr. Rochelle P. Walensky endorsed recommendations for a third dose of the Pfizer COVID-19 vaccine for certain at-risk groups, while overriding advisors' decision not to recommend a booster for other groups.

NBC News reports Dr. Walensky signed off on the recommendations for a booster shot of the Pfizer-BioNTech vaccine after a committee of advisors approved recommendation to people 65 years and older and residents in long-term care setting.

However, Dr. Walensky also endorsed booster shot reccommendations for individuals under 65 working in high-risk settings and institutional settings, overriding the committee's vote against such reccomendations on Thursday.

"This updated interim guidance from CDC allows for millions of Americans who are at highest risk for COVID-19 to receive a Pfizer-BioNTech COVID-19 booster shot to help increase their protection," the agency said in a statement obtained by NBC News.

The CDC is now recommending individuals 65 years and older, residents in long-term care settings and people between the ages of 50 and 64 with underlying medical conditions get a booster.

Individuals between the ages of 18 and 49 with underlying medical conditions, as well as those between 18 and 64 with an increased risk due to an occupational or institutional setting, may also be eligibile for a third shot at their own discretion.

Dr. Walensky said her decision to recommend a booster to individuals in at-risk work and other enviroments mirrors the Food and Drug Administration's recent emergency use authorization for a third dose of the Pfizer vaccine.

The CDC also plans to review data before making reccomendations about who will be eligible to receive the Moderna and Johnson & Johnson vaccines.

Moderna has already submitted its application to the FDA, while Johnson & Johnson has provided data, but has not specified when it will seek emergency use authorization for its booster shot.