Around half a million people who have a severely weakened immune system are to be offered a third COVID-19 jab.
This extra dose will be offered to anyone over 12 who was severely immunosuppressed at the time of their first or second dose, including those with leukaemia, advanced HIV, and recent organ transplants.
The recommendation, made by the Joint Committee on Vaccination and Immunisation (JCVI), is separate from a potential booster programme that would cover a larger proportion of the population.
While a booster would be designed to extend the already strong protection someone has received from their first and second jabs, this third dose is for people whose compromised immune systems mean two are insufficient.
People with severe immunosuppression are more likely to be seriously ill if they catch COVID-19.
Immunosuppression varies in severity and duration, and many of those who suffer from it also have lower levels of antibodies after coronavirus vaccination than the general population.
Data from the so-called OCTAVE trial showed almost everyone who was immunosuppressed mounted an immune response after two doses, but in around 40% of people, the levels of antibodies were low.
The JCVI said that while studies into how a third dose affects immunosuppressed people are ongoing, it is unlikely to cause harm and may increase protection.
Professor Wei Shen Lim, the chair of COVID-19 Immunisation for the JCVI, said: "We want people with severely suppressed immune systems to have the best chance of gaining protection from COVID-19 via vaccination.
"Therefore, we are advising they have a third vaccine dose on top of their initial two doses, as we hope this will reduce their risk of severe outcomes such as hospitalisation and death."
The JCVI advises that adults should get either the Moderna or Pfizer vaccine for the third dose, while the latter is preferable for those aged 12 to 17.
Health Secretary Sajid Javid has accepted the JCVI's advice and said the NHS would contact people "as soon as possible" to arrange appointments.
As for the aforementioned booster programme, Mr Javid said the plan remained for that to begin this month. Who a booster jab should be offered to will again be recommended by the JCVI.
Third doses should be given at least eight weeks after the second dose, the JCVI has said.