Advisers to the US Food and Drug Administration (FDA) voted on Friday (Sept 17) to recommend Covid-19 vaccine booster shots for Americans 65 and older and those at high risk of severe illness, after overwhelmingly rejecting a call for broader approval.

The panel also recommended that the FDA include healthcare workers and others at high risk of occupational exposure to the virus that causes Covid-19, such as teachers.

Despite the narrowed scope of the proposed authorisation, the panel's recommendation would cover most Americans who got their shots in the earliest stages of the US vaccination campaign.

"Today was an important step forward in providing better protection to Americans from Covid-19," White House spokesperson Kevin Munoz said. "We stand ready to provide booster shots to eligible Americans once the process concludes at the end of next week," he said.

The FDA is expected to make its decision on the third round of shots soon. It is not bound by the panel's recommendation but will take it into consideration.

Dr Paul Offit, an infectious disease expert at the University of Pennsylvania and a member of the panel, said the recommendation was "a step back" from the Biden Administration's recommendation of widespread boosters starting Sept 20.

"That is not this. This is: 'We're going to test the water, one foot at a time,'" said Offit, who voted "no" on the initial question of widespread use of boosters and "yes" on the more modest offering of vaccines for those at higher risk.

By a vote of 16 to two, members of the FDA's Vaccines and Related Biological Products Advisory Committee declined to recommend a third dose of the Pfizer/BioNTech vaccine to anyone age 16 and older who received their second shot at least six months earlier.

The panelists suggested the evidence supporting broad approval was inadequate, and they wanted to see more safety data, especially concerning the risk of heart inflammation in younger people after vaccination.

The panel members then unanimously approved the recommendation for a third shot for older and other high risk Americans, convinced by evidence showing they were at higher risk of severe Covid-19 and may be more likely to have waning immunity after the first rounds of shots.

Advisers to the US Centres for Disease Control and Prevention (CDC) are scheduled to meet on Sept 22 and 23 to discuss further recommendations for who will be eligible for the shots.

"We wonder how well this restriction can be managed in real-world or how to stop people, but the point is – it will be available for those that need it," said Jefferies analyst Michael Yee.

Pfizer could still come back with more data to support a wider authorisation or approval.

The drugmaker said it "will work with the FDA following today's meeting to address the committee's questions, as we continue to believe in the benefits of a booster dose for a broader population."

Pfizer shares closed down 1.3 percent, while BioNTech's US-traded shares finished the day 3.6 percent lower.

Some countries, including Israel and the UK, have already begun Covid-19 booster campaigns. The United States authorised extra shots for people with compromised immune systems last month and some two million people had already received a third shot, according to the CDC.

President Joe Biden has pushed for the additional shots in the face of surging hospitalisations and deaths caused by the highly contagious Delta variant of the coronavirus, mostly among the unvaccinated, and rising cases of breakthrough infections among fully vaccinated Americans.

Recent polls have shown that most vaccinated Americans want a booster shot to enhance their immunity.