WASHINGTON (REUTERS) - A United States Centres for Disease Control and Prevention (CDC) advisory panel on Thursday (Sept 23) recommended a booster shot of the Pfizer-BioNTech Covid-19 vaccine for Americans aged 65 and older and some adults with underlying medical conditions that put them at risk of severe disease.

But the panel declined to recommend boosters for younger adults, including healthcare workers, who live or work in institutions with high risk of contracting Covid-19, which could narrow the scope of the US Food and Drug Administration's (FDA) emergency use authorisation issued on Wednesday.

The guidelines voted on by the CDC's Advisory Committee on Immunisation Practices still need to be signed off on by agency Director Rochelle Walensky.

The recommendations are not binding, and states and other jurisdictions could disregard them and use other approaches to administering the booster shots.

Still, the vote by the group, following the FDA's authorisation clears the way for a booster roll-out to begin as soon as this week for millions of people who had their second dose of the Pfizer shot at least six months ago.

Beyond older Americans, the committee also recommended the shots for all adults over 50 with underlying conditions, as well as some 18- to 49-year-olds with those conditions, based on their individual risk profile.

Those conditions include cancer, diabetes, certain heart conditions and chronic kidney disease and lung disorders.

The recommendations only cover people who received their second Pfizer-BioNTech shot at least six months earlier. The CDC said that group is currently about 26 million people, including 13 million age 65 or older.

The panel gave the thumbs down for now to additional doses for groups including healthcare workers, teachers and residents of homeless shelters and prisons, in part because of the difficulty of implementing such a proposal.

Panel member Lynn Bahta, who works with the Minnesota Department of Health, voted against that measure, which would have broadly increased availability.

She said the data does not support boosters in that group yet. "The science shows that we have a really effective vaccine," she said. The committee said it could revisit the guidance later.

Last month, US President Joe Biden and eight top health officials said they hoped to start booster shots the week of Sept 10 for all people eight months after they had been vaccinated, saying that emerging data showed immunity wanes over time.

A group of independent FDA advisers last week overwhelmingly rejected recommending a broader approval of booster doses and instead recommended a narrower authorisation.

Dr Paul Offit, a member of the FDA advisory panel, said he believed the CDC advisers were worried that recommending boosters based on employment would make it difficult to limit who got them at a time where there was not enough data, especially about the effects of a third shot on younger people.

"My sense was that the committee felt that that was a hole that you could drive a truck through, that essentially what we were doing was basically what the (Biden) administration initially asked - to just have a vaccine for the general population, because obviously the pharmacists aren't going to figure out whether you're working in a grocery store or hospital," he said.

More than 180 million people in the US are fully vaccinated, or about 64 per cent of the eligible population.

Pfizer - and some top US health officials like Dr Anthony Fauci - have argued that the extra round of shots are needed to address waning immunity.

Dr Fauci and others have also said they could help contain surging hospitalisations and deaths caused by the highly transmissible Delta variant of the coronavirus by cutting breakthrough infections of fully vaccinated people.

The US is averaging about 128,000 new Covid-19 cases per day, off an August high of about 160,000.

The recent wave of US Covid-19 hospitalisations, primarily among the unvaccinated, may also have peaked, although the nation was still recording some 1,500 Covid-19 deaths a day over the past week, according to CDC data.

Some countries, including Israel and Britain, have already begun Covid-19 booster campaigns. The US authorised extra shots for people with compromised immune systems last month and around 2.3 million people have already received a third shot, according to the CDC.