The NHS in England is launching a spring booster vaccine campaign against Covid-19 for people most at risk of serious illness from the disease.
Around five million are eligible, including people aged 75 and over, some people with weakened immune systems, and older residents in care homes.
From Monday, older adults in care homes are expected to begin receiving their vaccines, given by visiting NHS teams.
Other eligible people will be able to receive jabs from mid-April.
Bookings for those appointments will open on Wednesday 5 April.
The rollout follows advice from the Joint Committee on Vaccination and Immunisation (JCVI), published in March.
The committee said, over the course of the pandemic, data available from the UK and internationally showed older people were more likely to experience severe disease. As a result, they would gain the most from protection from an additional vaccine dose this spring.
NHS director of vaccinations and screening, Steve Russell, said: "As a society we are learning to live with Covid, but for many it is still a virus that can cause serious illness and hospitalisation, and so it is still really important that those at greatest risk come forward and boost their protection in the coming weeks.
"There are still around 8,000 people in hospital with Covid according to the latest data, and the NHS has now treated more than one million Covid in-patients since the pandemic began.
He added: "So if you are over 75 or you have a weakened immune system, please come forward as soon as possible to book a Covid vaccine this spring, so you can enjoy summer with peace of mind."
Some people aged five and over who are defined as immunosuppressed will be among those offered a booster jab.
They include people who have had organ transplants or who have blood cancer, and those undergoing chemotherapy treatment for cancer.
Millions of invitations will be sent through the NHS App, alongside texts messages and letters for those without the app or not actively using it.
People who are eligible for jabs should make sure appointments take place at least three months after their last dose.
Vaccines will be available in around 3,000 sites across England, with the majority of jabs given in pharmacies and GP surgeries.
The last spring booster appointments will be available on 30 June.
Those eligible in Wales will be offered booster appointments between 1 April and 30 June.
In Scotland, the spring vaccine rollout will begin with people living in care homes. The jab will be offered to over-75s from 11 April, and anyone aged five and over with a weakened immune system from 24 April.
In Northern Ireland spring boosters will be available from 12 April. Most people will receive invites through their GP.