A round half a million people who have severely weakened immune systems will be offered a third dose of a Covid vaccine.

People with conditions including leukaemia, advanced HIV and recent organ transplants who are over the age of 12 will be eligible for a third jab.

The Joint Committee on Vaccination and Immunisation (JCVI) said the announcement is separate to any decision on a booster programme.

Health Secretary Sajid Javid said the government was continuing to plan for a booster programme to begin this month.

The JCVI said it was still deliberating on the potential benefits of booster vaccines for the rest of the population.

The Joint Committee concluded people who were severely immunosuppressed at the time of their first or second dose of vaccine may not have been able to mount a full response to vaccination.

Professor Wei Shen Lim, 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 recommendation does not apply to all those considered clinically extremely vulnerable. It is estimated to include up to 500,000 people.

Mr Javid said: “We know people with specific conditions that make them particularly vulnerable to Covid-19 may have received less protection against the virus from two vaccine doses. I am determined to ensure we are doing all we can to protect people in this group and a third dose will help deliver that.

“This is not the start of the booster programme – we are continuing to plan for this to begin in September to ensure the protection people have built from vaccines is maintained over time and ahead of the winter.”