The decision came a day after the U.S. Food and Drug Administration (FDA) authorized the booster shots.

According to CDC recommendations, for individuals who received a Pfizer-BioNTech or Moderna COVID-19 vaccine, some groups are eligible for a booster shot at six months or more after their initial series, including people 65 years and older; people 18 years and older who live in long-term care settings, who have underlying medical conditions, and who work or live in high-risk settings.

For the nearly 15 million people who got the Johnson & Johnson COVID-19 vaccine, booster shots are also recommended for those who are 18 and older and who were vaccinated two or more months ago.

The CDC now has booster recommendations for all three available COVID-19 vaccines in the United States. The agency also allowed "mix and match" dosing for booster shots.

People may choose the vaccine type that they originally received, or get a different booster.

A recent study from the U.S. National Institutes of Health (NIH) found the "mix and match" approach safe and effective, and that people who received the Johnson & Johnson vaccine produced stronger antibody levels after they got booster shots made by Moderna or Pfizer, compared to boosters from Johnson & Johnson.

"These recommendations are another example of our fundamental commitment to protect as many people as possible from COVID-19," CDC Director Rochelle Walensky said in a statement on Thursday evening.

"The evidence shows that all three COVID-19 vaccines authorized in the United States are safe -- as demonstrated by the over 400 million vaccine doses already given. And, they are all highly effective in reducing the risk of severe disease, hospitalization, and death, even in the midst of the widely circulating Delta variant," she said.

On Wednesday, the FDA authorized the use of a single booster dose of the Moderna COVID-19 vaccine at least six months after completion of primary shots for people 65 years of age and older, 18 through 64 years of age at high risk of severe COVID-19, and 18 through 64 years of age with frequent institutional or occupational exposure to SARS-CoV-2.

As for the Johnson & Johnson booster dose, the FDA authorized the use of a single booster dose at least two months after completion of the single-dose primary regimen to individuals 18 years of age and older.

About 189.9 million people have been fully vaccinated against COVID-19 as of Thursday, accounting for 57.2 percent of the whole population, according to CDC data. About 11.6 million people have received a booster dose by Thursday. — Agencies