Vulnerable people will be at risk over Christmas due to the insufficient pace of England's COVID booster rollout, a mathematics expert has warned.
John Roberts, from the Institute and Faculty of Actuaries, said six million people who are eligible for a COVID booster jab in England have not yet had one.
He warned the increase in the number of third doses given out has stalled in recent days, meaning booster coverage for the eligible priority groups will not be complete under mid-January.
So far, 6,826,159 people have had a booster vaccine in England since the rollout began on 16 September.
This is roughly six million short of the number of over 50s and vulnerable people who had their second dose more than six months ago, making them eligible for a third dose.
Roberts told BBC Radio 4’s Today programme: "My worry is that, at the current rate, we’re still not going to complete the first priority groups one to nine until mid-January.
“And that’s going to mean that there’s going to be a lot of mixing over Christmas with festive behaviour before and then on the day of people who haven’t got that extra protection from the booster, which really does make a huge difference."
Research suggests the protection offered by the Pfizer and AstraZeneca vaccines wanes over six months by a considerable level.
Protection against death for over 16s provided by the AstraZenca vaccine reduced from 94.1% to 78.7%, according to Public Health England.
Protection against death from the Pfizer vaccine is more robust, decreasing from 98.2% to 90.4%.
Boosters are now being given at walk-in vaccination sites with no appointment needed after concerns were raised about the pace of the rollout.
Watch: Walk-in sites now offering coronavirus booster jabs
From Monday, anyone in one of the eligible groups who had their second dose of a coronavirus vaccine at least six months ago can turn up at one of hundreds of jabs sites across England.
People are advised to use the NHS online walk-in finder to check where their nearest centre is.
NHS England said almost every person registered with a GP practice lives within 10 miles of a fixed vaccination site.
Dr Nikki Kanani, GP and deputy lead for the NHS COVID-19 vaccination programme said: “NHS staff are making it as easy as possible for people to get their top-up vaccination, and from today people can now go online, find their nearest site and go and get their booster without delay.
“The booster is not just a nice to have, it is really important protection ahead of what we know will be a challenging winter.
“So if you are eligible, please do check the site finder and go get your jab.”
Who is eligible for a COVID booster jab?
You are eligible for a booster if you had your second dose more than six months ago, and you are in one of the following groups:
people living in residential care homes for older adults
all adults aged 50 years or over
frontline health and social care workers
people aged 16-49 with underlying health conditions that put them at higher risk of severe COVID-19
adult household contacts of immunosuppressed individuals
Watch: Coronavirus in numbers – UK death toll rises to 140,632