People in Britain must come forward for Covid-19 vaccines when they are eligible if new restrictions this winter are to be avoided, Prime Minister Boris Johnson has said.

He was speaking as Britain's booster programme for Covid-19 vaccinations is to be expanded to younger people.

The UK government's Joint Committee on Vaccination and Immunisation (JCVI) said all healthy adults aged 40-49 should be offered a booster, six months after their second dose.

Previously, the booster programme was limited to those aged 50 and over, and the clinically vulnerable. Some 12.6 million Britons have received a third jab.

"If we want to avoid new restrictions on our daily lives, we must all get vaccinated as soon as we are eligible," Mr Johnson said at a news conference after the booster vaccination programme was expanded.

"Please go and get vaccinated to protect yourself and others, and, in doing so, we can help to ensure that we can continue in the way that we are: sticking to our plan of using vaccination to control this virus."

The JCVI also said that 16 and 17-year-olds should now be offered a second dose of the Pfizer/BioNTech jab.

That age group had previously been offered only one dose in Britain, pending more data on the jab's safety in under-18s.

The expanded age groups covered by the new advice will "help extend our protection into 2022", Professor Wei Shen Lim of the JCVI said in a statement.

People aged under 40 might be advised to get a booster in the future, Prof Lim said.

"It may well be that adults who are under 40 years might require a booster dose or a third dose at some point.

"We don't know whether that is definitely the case yet.

"We are looking very closely at the data all the time and should there be sufficient signal to warrant a third dose, so a booster dose for this age group, then certainly we will announce that and advise that accordingly."

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Health minister Sajid Javid said: "Our Covid-19 vaccination rollout has been a phenomenal success, saving countless lives, reducing pressure on the NHS and helping us stop the spread of the virus.

"We are expanding the programme even further and today I have accepted the advice from the independent experts at the Joint Committee on Vaccination and Immunisation (JCVI) to extend the additional offer of a booster jab to people aged 40 and over and offer a second dose of a vaccine to all young people aged 16 to 17 as part of the primary vaccination schedule.

"All four parts of the UK intend to follow the JCVI's advice.

"I have asked the NHS to prepare to offer those eligible a vaccine as soon as possible.

"We know immunity to Covid-19 begins to wane after six months and new data published today shows a third dose boosts protection against symptomatic infection to more than 90% - this highlights just how important it is that everyone eligible gets their top-up jabs as soon as possible.

"The JCVI will keep under review whether the booster programme should be extended to all people under the age of 40 and I look forward to receiving their advice in due course.

"This is a national mission - the vaccines are the best way to protect yourself and your loved ones and I urge everybody to get your jabs as soon as you can."

So far, nearly 143,000 people have died in Britain within 28 days of a positive test for Covid, the second-worst toll in Europe behind Russia.

Meanwhile, a study by the UK Health Security Agency has show that booster jabs give more than 90% protection against symptomatic infection in adults aged over 50.

The findings of the new research show that two weeks after receiving a booster dose, protection against symptomatic infection was 93.1% in those who had initially received Oxford/AstraZeneca, and 94.0% for Pfizer/BioNTech.

After two doses of either vaccine, effectiveness against symptomatic disease appears to wane over time.

While experts say vaccine effectiveness against severe outcomes, such as hospital admission, remains high for several months after completing the primary course, researchers have seen greater waning in older adults and those with underlying medical conditions compared with young, healthy adults.

Additional reporting PA