Letters are being sent to those aged over 65 and the clinically vulnerable to invite them to receive the first dose of their Covid-19 vaccine.
It comes a day after the UK reached the target of giving at least one dose of the vaccine to 15 million people, the majority of them most at risk from the disease.
This means that the first four priority groups, those aged over 70 and the clinically extremely vulnerable have all been invited to receive the first dose of the vaccine.
A small number of just over 537,000 have received the second and final dose, meaning they have maximum protection from the virus.
Some 1.2 million invitations have been sent to the over-65s and the clinically vulnerable, with a similar number of invitations expected to be sent this week.
People who receive a letter will be able to choose where they receive their vaccine from more than 100 large vaccination centres or almost 200 pharmacy services.
The Joint Committee on Vaccination and Immunisation defines clinically vulnerable people as those with conditions including chronic respiratory disease such as chronic obstructive pulmonary disease (COPD), cystic fibrosis and severe asthma.
The government is aiming to vaccinate the 17 million in groups five to nine by the end of April, something that will be done alongside administering second doses for many in the first four groups.
In a video message on Sunday, Prime Minister Boris Johnson said there was still “a long way to go” and that there would “undoubtedly be bumps in the road”.
Meanwhile, Mr Johnson will outline on February 22, how the country will exit the current lockdown, as pressure grows from some Conservative MPs to reopen sectors such as hospitality and tourism.
Over the weekend, more than 60 Tory backbenchers are said to have backed a Covid-19 Research Group letter calling on the PM to commit to a firm timetable, starting with the reopening of schools on March 8, and ending with the lifting of all legal restrictions by the end of April.
Mr Johnson appears more cautious, however, saying the data would need to be studied “very, very hard” for evidence that the vaccines are driving down the number of daily cases and deaths.
He has said he does not want to be forced into a “reverse ferret” if restrictions are eased too fast, causing a resurgence.
Chris Hopson, chief executive of NHS Providers, said: “We must not let down our guard in our fight against the virus.
“There is still a lot to understand before we can be wholly certain of the impact of the vaccination programme.”
On Sunday, Mr Johnson had hailed 15 million vaccinations as a “truly national UK-wide effort”, adding: “We’ve done it together. And I want to thank each and every person who has helped make it happen.”
The 15,062,189 who have had a first dose is the equivalent of 22.5% of the total UK population, and 28.6% of people aged 18 and over.
Vaccines minister Nadhim Zahawi said on Twitter that the government would “not rest” until the vaccine was offered to all over-50s by the end of April, and then all adults.
Also on Sunday, the UK recorded a further 258 Covid-19-related deaths, and 10,972 new cases – down from the 621 deaths and 13,308 cases recorded on Saturday.