Health officials have warned of a “bumpy few months ahead” as they extended the Covid-19 booster programme to include healthy adults over the age of 40.

Ministers across the UK have accepted a recommendation from the Joint Committee on Vaccination and Immunisation (JCVI) that the booster programme should be extended to include people aged 40 to 49.

Second doses for 16 and 17-year-olds have also been approved after the JCVI said this group should be offered a second jab of the Pfizer/BioNTech jab 12 weeks after they received the first.

Read more: Boris Johnson to make Covid announcement today - what time and how to watch

The JCVI said that the broadening of the booster campaign and the offer of a second jab to 16 and 17-year-olds will “help extend our protection into 2022”.

Why do we need to get our booster jabs? 

Prime Minister Boris Johnson urged people to get their booster to get the extra level of protection which will make “all the difference to winter, to Christmas”.

And England’s deputy chief medical officer Professor Jonathan Van-Tam said that everyone has a role to play in helping the UK have “as safe and disruption-free a winter as possible”.

Speaking on a visit to a medical centre in east London, Mr Johnson said: “What’s happening is if you can get your booster then your immunity goes right back up to 95%.

“So far we’ve got 75% of everybody over 70 getting a booster, there’s a huge number of people, but it’s that further 25% that will make all the difference to winter, to Christmas, to our plans going forward, because it’s that extra level of protection that we really need.

“So the message is: anybody over 70, come forward, get your booster; anybody over 50, come forward and get your booster now; in the next week or so, anybody over 40 as well, come forward and get your booster.”

It comes as a new study highlighted how boosters can significantly increase people’s protection against getting a symptomatic case of Covid-19.

Read more: If you have Covid you 'will no longer have to self-isolate' under new plans

When will the booster be available? 

The NHS has been asked to prepare to offer those eligible a booster vaccine as soon as possible.

So far, some 12.6 million people have had a third Covid-19 jab.

What vaccine will I have? 

The JCVI said people should be offered the Pfizer or Moderna jab as a booster, irrespective of which vaccine they had initially.

The booster will be offered six months after the second vaccine was given, with people able to book their appointment after five months.

What difference does the booster make? 

Two weeks after getting their booster, adults over 50 had at least 93% reduced risk of getting a symptomatic case of Covid-19, according to a study from the UK Health Security Agency (UKSHA).

Protection against more severe disease and death is expected to be even higher.

Professor Wei Shen Lim, chair of Covid-19 immunisation for the JCVI, said: “The booster dose markedly strengthens existing protection and will extend the duration of that protection against serious disease.

“We therefore urge people who are eligible for a booster to step forward and have your booster and maximise your protection.”

He added: “Booster vaccine doses in more vulnerable adults, and second vaccine doses in 16 to 17-year-olds, are important ways to increase our protection against Covid-19 infection and severe disease.

“These vaccinations will also help extend our protection into 2022.”

What challenges are we facing?

Prof Van-Tam said that the nation was in a period of “near-term danger” but he hoped for “calmer waters” by spring.

He told the briefing: “I regard it as still a time of great delicacy and quite a bit of near-term danger.

“Do I personally frame that if our booster programme goes well and vaccine uptake remains high, that we will be in a much set of calmer waters by the middle of spring? then I think I do. But this virus is unpredictable.

“But I do see, once the spring gets here, hopefully some calmer waters ahead.”

He also made a plea for unvaccinated people and those who had only had one vaccine to come forward to ensure they are protected.