The British government has warned of Christmas Covid-19 restrictions as it urges people to have booster shots amid waning resistance.
Health Secretary Sajid Javid said the drive to get people to have the top-up was a “national mission”.
Meanwhile, the chief medical adviser at the UK Health Security Agency said the number of deaths among the vaccinated population is rising as protection weakens.
Ministers are also considering changing the rules so those who have not received the booster shot may face travel restrictions.
Mr Javid said younger relatives should urge eligible parents and grandparents to have a booster and the flu vaccine.
He said that if “we all come together and play our part”, the country could “avoid a return to restrictions and enjoy Christmas”.
“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," Mr Javid said.
“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.
“And if you haven’t yet had your first and second vaccines, it is not too late. The NHS will always be there to welcome you with open arms.”
Official guidance was updated this month to say the government “is reviewing the implications and requirements of boosters for international travel” and “looking at whether and how booster vaccinations could be included in the NHS Covid Pass for travel”.
But the Mail on Sunday reported that officials are divided over how soon to implement the restrictions and are discussing a grace period.
That would allow people to continue to travel without quarantine if they have sought a booster six months after their second shot but have not yet been given an appointment.
The chief medical adviser at the UK Health Security Agency, Dr Susan Hopkins, said elderly and vulnerable people who are double vaccinated have started dying because Covid-19 vaccines' efficacy was waning.
“We’re still seeing deaths in mainly the unvaccinated population but increasingly, because of immune-waning effects, there are deaths in the vaccinated group as well," she said.
“It is particularly the older age groups, so the over-70s in particular, but also those who are clinically vulnerable, extremely vulnerable and have underlying medical conditions.”
People over 50 and those most at risk from Covid-19 are eligible for a booster six months after their second shot.
NHS figures from Sunday show that more than seven in 10 people aged 80 and over have had a booster, while almost three in five people aged 50 and over have had one, the Department of Health and Social Care said.
Three million more people in England are being invited to have their booster shots next week, after Saturday saw a record day with more than 371,000 people receiving theirs.