Boris Johnson has repeated his call for those eligible to get their COVID-19 booster jabs over the coming weeks, insisting vaccines are the UK’s “way through” the pandemic.

The prime minister has resisted calls from health leaders to implement the government’s Plan B for managing rising infection rates this winter, which would see tighter restrictions brought in to ease pressure on the NHS.

Health Secretary Sajid Javid admitted this week that cases could reach 100,000 a day, but Downing Street insisted there was spare capacity in the NHS and that Plan B would only be activated if it came under “significant pressure”.

Plan B includes advising people to work from home and the mandatory use of face masks.

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Mr Johnson, who has said there are no plans for another lockdown, said: “Vaccines are our way through this winter.

“We’ve made phenomenal progress, but our job isn’t finished yet, and we know that vaccine protection can drop after six months.

“To keep yourself, your loved ones, and everyone around you safe, please get your booster when you get the call.

“We can and will beat this virus, but only if we listen to the science and look out for each other.

“This is a call to everyone, whether you’re eligible for a booster, haven’t got round to your second dose yet, or your child is eligible for a dose – vaccines are safe, they save lives, and they are our way out of this pandemic.”

Experts have criticised the booster programme and jab rollout to children aged 12 to 15 for being too slow.

Four million booster doses have been administered, with some signs of the rollout speeding up, as half a million were booked over Wednesday and Thursday alone.

This week has been the busiest so far for the National Booking Service since the booster campaign began.

Number 10 has said: “As set out in the autumn and winter plan, the winter months will lead to the increased transmission of viruses.

“Vaccines are our best line of defence, but data shows that the natural immunity provided by vaccines will wane over time, particularly for older adults and those more at risk from COVID.

“Recent studies suggest protection against death falls from 95% to 80% for AstraZeneca after six months, and from 99% to 90% for Pfizer.

“The booster programme is designed to top up this waning immunity for those most at risk over the winter months. A 15% drop in efficacy could lead to many more avoidable deaths and cases of severe illness from COVID.

“Early results from Pfizer shows that a booster dose can increase the protection from our vaccines back up to 95.6% against symptomatic infection.

“This additional protection is vital, and everyone aged over 50 or who is at high risk from COVID will be invited for their booster jab six months after their second dose.”

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On Saturday, a government scientific adviser called for tougher COVID restrictions now and not later, warning the country is “dilly-dallying into lockdown”.

Professor Stephen Reicher told Sky News that vaccines are “not quite enough” on their own and “other protections” are needed now to tackle coronavirus.

His concerns have been echoed by fellow government adviser, Professor Peter Openshaw, who said he was “very fearful” of another “lockdown Christmas”.

Professor Openshaw, a member of the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), said case numbers and death rates are currently “unacceptable”.

Daily cases topped 50,000 on Thursday for the first time since 17 July. Although, Saturday saw a slight drop with 44,985 infections reported compared to 49,298 on Friday.

Similarly, official figures show 135 deaths were recorded on Saturday, compared to 180 the day before.

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Speaking in a personal capacity, Professor Openshaw told BBC Breakfast: “I’m very fearful that we’re going to have another lockdown Christmas if we don’t act soon.

“We know that with public health measures, the time to act is immediately. There’s no point in delaying.

“If you do delay, then you need to take even more stringent actions later. The immediacy of response is absolutely vital if you’re going to get things under control.

“We all really, really want a wonderful family Christmas where we can all get back together.

“If that’s what we want, we need to get these measures in place now in order to get transmission rates right down so that we can actually get together and see one another over Christmas.”

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Elsewhere, the World Health Organization has warned the vaccine alone will not be able to lift the world out of the pandemic.

Spokesperson Margaret Harris told Times Radio: “The problem is focusing on one thing, the vaccine isn’t going to get us out of this. We really have to do other measures.

“We have got to be serious about not crowding. We have still got to be looking at wearing the masks, when you’re indoors particularly.”