Up to 40% of Covid-19 vaccine appointments are no-shows, health leaders have warned, as the government said it has met its target of offering all adults a jab by the end of the year.
The NHS Confederation said it was "encouraging" to see people coming forward for their jabs, but it had heard reports that some vaccination clinics were only a third full as cases of the Omicron variant continue to rise rapidly.
It remains a concern going in to the New Year, with case numbers reaching record numbers each day and the number of patients in hospital with coronavirus in England having climbed to its highest level since February.
Coronavirus: What you need to know
Chief executive Matthew Taylor said some primary care leaders had told the organisation that people were not turning up to as many as 40% of their booked appointments.
It is thought some of the no-shows may be due to people not cancelling their appointment if they get a vaccine sooner at a walk-in centre or they cannot attend as they have tested positive for Covid.
"Cases of Omicron are rising rapidly… Health leaders are worried about the level of illness and demand that their staff across the NHS could have to respond to in January and so, it is vital that everyone who is eligible takes up the offer of a jab or booster shot," he warned.
"Some primary care leaders are telling us their vaccination clinics are only a third full and that people are not turning up to as many as 40% of their booked appointments.
"While walk-ins continue to be on offer, they are calling on people to stick to their scheduled vaccine appointments and if they can’t for whatever reason that they cancel with notice as this will help manage pressures, particularly given the staffing crisis we are facing currently."
Anyone who has not yet received the vaccine is now being urged to come forward to curb the spread of the disease.
The Department of Health and Social Care (DHSC) announced on Friday that it had met the goal set by the prime minister to ensure all eligible over-18s in England had the chance to get their vaccines by December 31.
Health Secretary Sajid Javid said he was "delighted" to confirm the booster target had been hit.
"I am incredibly proud of the work the NHS has done to accelerate the programme and offer my thanks to the frontline staff, volunteers, Armed Forces and British public who have made it possible for us to meet this commitment," he said.
Shadow health secretary Wes Streeting said: "While eagle-eyed observers will note that the prime minister shifted the goalposts on what he actually promised to deliver, this should not detract one bit from the fact that the NHS has pulled off an unprecedented number of vaccinations in recent weeks.
"Everyone involved should be enormously proud of what they’ve achieved, from volunteer vaccinators through to members of our armed forces drafted in to help.
"I urge everyone who has yet to receive their first, second or booster jab to book without delay."
Did the government 'shift the goalposts' on booster jab promise?
The government set a target at the end of November to offer a booster jab to everyone eligible aged 18 and over by the end of January 2021.
But this goal was subsequently brought forward by a month in response to the emerging threat of the new variant sweeping across the country.
Boris Johnson said on December 12 that it was clear two doses of the vaccine were "simply not enough" to give the level of protection needed and that, without a boosted population, the NHS could be overwhelmed.
He therefore set a new deadline of offering every adult in England a booster by December 31.
It's understood the claim to have achieved the goal is measured based on not just the actual take-up of boosters but also those eligible having been contacted.
The Welsh government also announced on Friday a booster vaccination appointment had been offered to every eligible adult in the country.
A total of 435,293 booster and third doses of Covid-19 vaccine were reported in the UK on Wednesday, the highest daily figure since December 23.
Figures published by the UK’s four health agencies show more than 33.5 million booster and third doses have now been delivered in the UK, with 1.8 million in the past seven days.
The NHS national booking system was opened to all people over the age of 18 on December 15.
Government data shows 58.3% of the population (aged 12 and over) have now had a booster or third jab.