Boris Johnson has warned it will take several "long, cold months" for the Covid-19 vaccine to protect the most vulnerable.

Speaking at today's Downing Street press conference, the Prime Minister said there were "immense logistical challenges" in distributing the vaccine.

It comes after the first vaccine from Pfizer/BioNTech was approved for clinical use by regulators, making the UK the first country in the world with a clinically approved vaccine ready to roll out.

The Prime Minister told a Downing Street press conference: "It will inevitably take some months before all the most vulnerable are protected - long, cold months.

"So it's all the more vital that as we celebrate this scientific achievement we are not carried away with over-optimism or fall into the naive belief that the struggle is over."

Despite warning against over-optimism, Boris Johnson said it was now "sure and certain" that life could start returning to normal in 2021.

A combination of community testing, vaccines and social distancing measures were still necessary, he added.

"As we do all this we are no longer resting on the mere hope that we can return to normal next year, in the spring, but rather the sure and certain knowledge that we will succeed and together reclaim our lives and all the things about our lives that we love," he said.

Simon Stevens, chief executive of NHS England, said the bulk of vaccinations will take place in "January through to March or April for the at-risk population".

Boris Johnson (C), Deputy Chief Medical Officer for England Jonathan Van-Tam (L) and Chief Executive of the NHS Simon Stevens at today's virtual press conference from 10 Downing Street

He told the Number 10 press conference: "Supplies from the manufacturer are phased so the initial tranche in December is going to enable us to get started but the bulk of this vaccination programme, either through this vaccine, or hopefully others as well that will join it, will take place in the period January through to March or April for the at-risk population.

"The majority of the early vaccinations will, as I say, be for the over-80s and for care home residents and since you need two jabs with an initial injection and then a booster given to you around 21 days apart that means that we've got to reserve the second dose for the people who are getting the first dose in December to make sure that that second dose is available for them."

Deputy chief medical officer Professor Jonathan Van-Tam echoed Johnson's words of caution, warning of a "tidal wave of infections".

“We have to be realistic about how long this is going to take," the professor said.

“It is going to take months, not weeks.

“And, for now, the other measures, the tier measures, the social distancing have to stay in place.

“If we relax too soon, if we just, kind of, go ‘oh, the vaccine’s here, let’s abandon caution’, all you are going to do is create a tidal wave of infections.

“And this vaccine has then got to work in a head wind to get back ahead of the game. And that will make it harder.”

Logistical complications in delivered the vaccine are numerous.

The delicate vaccine has to be moved around the country in a carefully controlled manner, and be stored initially at minus 70 degrees centigrade.

Simon Stevens said "there are a limited number of further movements that we are allowed by the regulator to make".

Chief Executive of the National Health Service Sir Simon Stevens speaks today

What's more, the packaging of the vials see 975 doses packaged together - meaning it cannot at this early stage be delivered to every individual GP surgery or pharmacy, as you would for, say, the flu jab.

Sir Stevens explained: "So the phasing of delivery, the way we will do it, is that next week around 50 hospital hubs across England will start offering the vaccine to the over-80s and to care home staff and others identified by the JCVI typically they may be people who were already down to come into hospital next week for an outpatient appointment.

“So if you are going to be one of those people next week or in the weeks that follow the hospital will get in touch with you, you don’t need to do anything about it yourself.”