Thousands of people under 50 are being called forward for Covid vaccines as households of those with a weakened immune system are to be prioritised for jabs.
The recommendation from the Joint Committee on Vaccination and Immunisation (JCVI), accepted by the Government, means some under-50s due to have their first doses delayed until at least May will now be able to access jabs.
People over the age of 16 who are living in a house with an adult with a weakened immune system - such as those with blood cancer, HIV or are having immunosuppressive treatment such as chemotherapy - will now be prioritised for their vaccine.
It is hoped the move will help to limit the spread to immunosuppressed adults, who have a weaker immune system and are less able to fight infections naturally.
The NHS in England has been told to offer jabs to this group after the Government accepted the recommendation.
People who are immunosuppressed are at high risk of Covid-19 and the JCVI said recent evidence suggests that they may not respond as well to the Covid-19 vaccine as others.
There is growing evidence that the vaccine may reduce the transmission of the virus, the advisers added.
As a result, the JCVI said that adults who live with immunosuppressed adults should be prioritised for their jab alongside people who have underlying health conditions which put them at a higher risk of Covid-19.
Professor Wei Shen Lim, chair of Covid-19 immunisation for the JCVI, said: "The vaccination programme has so far seen high vaccine uptake and very encouraging results on infection rates, hospitalisations and mortality.
"Yet we know that the vaccine isn't as effective in those who are immunosuppressed.
"Our latest advice will help reduce the risk of infection in those who may not be able to fully benefit from being vaccinated themselves."
Dr Mary Ramsay, head of immunisation at Public Health England, added: "Our surveillance systems and research studies are showing that the Covid-19 vaccines can reduce asymptomatic infection and limit transmission of the virus.
"By vaccinating those who live with adults who are immunosuppressed, we can further help protect vulnerable people."
In a letter to Prof Lim, Health Secretary Matt Hancock said: "I am asking NHS England and Improvement to take this advice forward and prioritise household contacts of the severely immunosuppressed for vaccination as you recommend.
"My officials have also shared your advice with colleagues leading the Covid-19 vaccines programmes in each of the four nations of the UK."
The JCVI has not made the same recommendation about family members of children who are immunosuppressed, or children under the age of 16 who live with immunosuppressed adults.
The vaccine rollout in the UK, which has reached more than 30million adults, will focus on second doses in April after a hit to expected supplies.
Adults under-50 will still receive a first dose by July 31, the Government maintains.
Boris Johnson on Monday announced that 60million doses of a new Novavax vaccine will be manufactured in the UK.
His comments came at the first Downing Street address from a new media briefing facility, where Chief Medical Officer for England Chris Whitty also said that the UK's vaccine protection as Covid rules are relaxed remains "leaky".
