The first people in the UK are expected to receive a coronavirus vaccine later, as rollout of the Pfizer/BioNTech jab begins in what is being called "V-day".
About 70 hospital hubs in the UK are gearing up to vaccinate the over-80s and some health and care staff, reports BBC.
It marks the start of a mass programme aiming to protect the most vulnerable and return life to normal.
More than 60,000 people in the UK have died after being infected with Covid, according to government figures.
The UK will be the first country in the world to start using this vaccine after regulators approved its use last week.
Vaccination will not be compulsory.
Prime Minister Boris Johnson said: "Today marks a huge step forward in the UK's fight against coronavirus."
But he added: "As the programme ramps up in the weeks and months ahead, it is as important as ever to keep to the Covid winter plan - following the rules in your area and remember the basics of hands, face and space."
Ministers have warned it could be Easter by the time restrictions are lifted in a significant way.
Health Secretary Matt Hancock said there was now "light at the end of the tunnel".
"We will look back on today, V-day, as a key moment in our fightback against this terrible disease," he added.
The government has secured 800,000 doses of this vaccine to start with, but orders have been placed for 40 million in total, enough for 20 million people as two courses are needed.
The majority of that is not expected to become available until next year, although government sources said another four million doses should arrive in the country by the end of the year.
The government had initially been promised 10 million doses by the end of the year, but problems with manufacturing mean the supply is going to be slower than originally hoped for.
NHS has made 'incredible effort'
Refrigerated containers holding the vaccine doses have been arriving in the UK over the past few days from Belgium, where it is made, and sent to the network of hospitals that will carry out the vaccinations.
Chris Hopson of NHS Providers, which represents hospitals, said there had been an "incredible effort" to start the vaccination programme so quickly given that the vaccine was only approved for use in the UK last week.
"This is our wonderful NHS in action," he added.
Hospital patients over the age of 80 are among the first people who will get the jab on Tuesday, along with the NHS staff who are carrying out the vaccinations.
Some of the most at-risk NHS staff will also be offered the vaccine and, in the coming days, care homes will be able to book their staff in for vaccination.
But rollout of the jab has been complicated by the need to store it at -70C and that it comes in packs of 975 doses, which cannot yet be split into smaller batches.
That has meant it has not been possible to offer it to care home residents in the first phase of rollout, despite the government's vaccine advisers designating them the highest priority.
The NHS is awaiting guidance from the drugs regulator, the Medicines and Healthcare Products Regulatory Agency, to see what steps are needed to allow these batches to be split and sent to individual care homes.
That is expected to come in mid-December, which will pave the way for the vaccine to be offered to care homes and distributed to more than 1,000 designated GP centres.
Mass vaccination centres at conference centres, sports stadiums and leisure centres are also expected to be established next year.
Who will get the jab eventually?
The hope is the bulk of the most vulnerable groups will be offered the jab in the first few months of 2021. But achieving that is likely to require approval of a second vaccine made by Oxford University and AstraZeneca.
The government has pre-ordered 100 million doses of that jab, which is easier to distribute because it does not need to be kept in ultra-cold storage and is made in the UK. The MHRA is currently assessing the data from trials on that vaccine.
Vaccination has been promised to all over-50s plus younger adults with underlying health conditions - about 25 million people in total.
The vaccine is given as two injections, 21 days apart, with the second dose being a booster. Immunity begins to kick in after the first dose but reaches its full effect seven days after the second dose.
Most of the side-effects are very mild, similar to the side-effects after any other vaccine and usually last for a day or so.
The vaccine was 95% effective for all groups in the trials, including elderly people.
But it is not yet known how long the immunity it provides lasts, or whether it stops people from passing the virus on to others.
Clive Dix, deputy chair of the government's Vaccine Taskforce, said: "We may have to vaccinate every year like we do for the flu."
But he said getting to this point was a "great achievement" as vaccine development could take 10 years, but had been achieved in 10 months.