Vulnerable patients and the over-80s need a third coronavirus vaccine "pretty soon", experts have warned.
Ministers have been urged to "stop hanging around" and launch a mass booster vaccination programme that could help protect 500,000 vulnerable people from coronavirus.
Third - or "booster" doses - will be offered to those who are classed as vulnerable ahead of a feared winter Covid surge.
Concerns of a fourth wave of coronavirus are mounting as health officials decide whether all over-50s should get the third vaccination.
The NHS was told in June to prepare to re-vaccinate 32 million people.
The programme was due to start next week, however it has not yet been given the go-ahead from the Joint Committee on Vaccination, an independent panel which advises the Government.
Countries such as the US and Israel are already offering all over-12s a third jab.
Professor Paul Hunter, from the University of East Anglia, said all over-80s and vulnerable patients needed a third jab quickly.
He told the Daily Mail : "The evidence is that these people won’t have responded that well to vaccine.
"They should be boosted and they should be boosted pretty soon."
The Commons Health Committee chair, Jeremy Hunt said: "The clear lesson for the UK seems to be to get on with booster jabs, not just for the clinically vulnerable but for everyone,’ added the Tory former health secretary.
"The latest study from King’s College London showed vaccine effectiveness dropping after six months, so why are we hanging around?"
Experts on the Joint Committee on Vaccination say they are still waiting for evidence before issuing final advice to ministers on a booster programme, including on "mix and match" jabs.
The Cov-Boost study is still examining the impact of a third dose on patients' immune responses.
There are fears that a fourth wave could hit the UK this winter, and experts believe third vaccinations could help curb hospitalisations.
On Tuesday, the Office for National Statistics revealed antibody levels are starting to decline in older adults, who were the first to receive both jabs.
It said 92 per cent of over-80s tested positive for antibodies in August, down from 95 per cent in May.
Ministers are still confident the JCVI will give the green light for a wider programme of booster jabs for the over-50s to begin at some point this month.
A source told the Daily Mail the decision focused on the details of the programme, such as whether to use a "mix and match" approach, rather than whether to go ahead.
The JCVI said its ongoing deliberations over boosters was separate to yesterday’s decision to offer extra doses to seriously vulnerable patients.
This roll-out follows trial data showing about 40 per cent of patients who are immuno-compromised fail to produce a good antibody response after two vaccine doses.
Sajid Javid said: "This is not the start of the booster programme – we are continuing to plan for this to begin in September to ensure the protection people have built from vaccines is maintained over time and ahead of the winter."
The elderly and people with underlying health conditions are likely to be included in any booster campaign.
