The U.S. Centers for Disease Control and Prevention (CDC) on Thursday recommended the COVID-19 vaccine boosters for recipients of the Moderna and Johnson & Johnson (J&J) shots, and said Americans can choose a different shot from their original inoculation as a booster.

CDC director Rochelle Walensky signed off on the recommendations late on Thursday, aligning the agency with the U.S. Food and Drug Administration's (FDA) authorization on Wednesday for the additional boosters and "mix-and-match" dosing.

Walensky said that vaccines authorized in the United States "are all highly effective in reducing the risk of severe disease, hospitalization, and death, even in the midst of the widely circulating Delta variant" of the coronavirus.

A panel of advisers to the CDC had unanimously backed COVID-19 vaccine boosters earlier on Thursday.

Still, health officials and public health experts said the booster rollout could be confusing. The panel struggled with trying to make the language of its recommendations as clear as possible, and also offer flexibility for patients to get a vaccine of their choice.

The recommendations also open the door for recipients of the one-shot J&J vaccine to get a dose of the Pfizer or Moderna vaccines that have been shown to afford greater protection in a variety of studies.

Although these vaccines have been highly effective in preventing serious illness and death, some government scientists have suggested that boosters are needed to keep immunity high, especially as the extremely contagious Delta variant can cause breakthrough infections among some who are fully vaccinated.

The Advisory Committee on Immunization Practices (ACIP) of CDC voted to recommend booster doses for everyone 18 and older who received a first dose of Johnson & Johnson's vaccine at least two months earlier. For those who received their second dose of Moderna's vaccine at least six months earlier, ACIP recommended a third shot for those age 65 and over, as well as some individuals at risk of severe illness and those at high risk of exposure to the virus through their jobs.

Dr Helen Talbot, an infectious disease expert at Vanderbilt University and a panel member, said she voted for the recommendation, but cautioned, "Moderna has very good vaccine efficacy. There will be some confusion with the higher versus lower dose."

Booster doses of the Moderna vaccine will be 50 micrograms, half the strength of the original shots.

"Those not at high risk should be really thoughtful about getting the higher dose. I do think we need to be very aware of potential complications," said Dr Talbot.

Experts said communicating those risks could be an issue, especially given the flexibility of allowing people to choose a different vaccine from their original series.

The FDA and CDC previously signed off on booster shots of the COVID-19 vaccine from Pfizer and partner BioNTech SE for the same groups included in the Moderna recommendations.

Pfizer released data on Thursday suggesting that a booster dose was highly protective for those age 16 and older compared with protection nearly a year after receiving the two-dose regimen due to waning efficacy over time.

About 11.6 million people have so far received a booster dose, according to data from the CDC.