Elderly and vulnerable people in Britain have been urged to get their COVID-19 booster shots as part of a "national mission" to help avoid a return to coronavirus restrictions over Christmas.

So far almost 10 million people in the U.K. have received a top-up jab, but around 30% of over-80s and 40% of over-50s in England are yet to receive a vaccine booster, according to the National Health Service (NHS).

Three million more people in England are being invited to have their coronavirus booster jabs next week.

People over 50 and those most at risk from COVID-19 are eligible for a booster six months after their second jab.

Health Secretary Sajid Javid said younger relatives should urge eligible parents and grandparents to take up the offer of a booster as well as to get the flu vaccine. He said that if "we all come together and play our part" then the country can "avoid a return to restrictions and enjoy Christmas."

"Almost 10 million people in the UK have received their COVID-19 booster and third jabs, a phenomenal achievement in under two months," Javid said. "As we approach this milestone, I want to thank those who have come forward and urge everybody across the nation to get vaccinated, get protected and get boosted.

"We know immunity begins to wane after six months, especially for the elderly and the vulnerable, and booster vaccines will top-up their protection to keep people safe over the winter," he added. "I strongly urge everybody who is eligible for a COVID-19 booster or flu vaccine to take up the offer as soon as you can."

Javid said that Britain "can get through this challenging winter" and called on citizens to help the government to "avoid a return to restrictions and enjoy Christmas."

The third jab could become a requirement to avoid travel restrictions in a move which may further drive take-up of boosters.

Official guidance was updated earlier this month to say the Government "is reviewing the implications and requirements of boosters for international travel certification" and "looking at whether and how booster vaccinations could be included in the NHS COVID-19 Pass for travel."

But the Mail on Sunday reported that officials are divided over how soon to implement the measures and are discussing a grace period to allow people to continue to travel without quarantine if they have sought a booster six months after their second jab but have not yet been offered an appointment.

NHS England will begin inviting those who had their second dose more than five months ago, meaning they can receive their top-up as soon as it has been six months since their last vaccine.

Government guidelines state that people who are 50 and over, or aged 16 and over with a health condition that puts them at high risk of contracting COVID-19, and had their second dose six months ago are eligible to get their booster.

NHS England said 8 million people were vaccinated by Saturday out of 12 million people that had been invited.

By next week, 16 million invitations for a booster will have been sent since the NHS booster program began.

Nikki Kanani, deputy lead for the NHS vaccination program in England, said: "Thanks to the hard work and non-stop efforts of NHS staff, the NHS COVID-19 vaccination program has now vaccinated more than 8 million people, ensuring they are protected ahead of winter.

"The NHS has invited millions of people who are eligible for a booster vaccine, and with hundreds of walk-in sites or appointments available online if you prefer, it is quick and easy to get vaccinated. I’ve had my booster and I would encourage anyone who is eligible to come forward as soon as possible to protect you and your loved ones."

More than 88 million vaccines have already been delivered and nine in 10 adults have had their first dose since Margaret Keenan received the first jab outside of a clinical trial in Coventry, in December 2020.