Around half a million people who have severely weakened immune systems will be offered a third dose of a coronavirus vaccine.

People with conditions such as leukaemia, advanced HIV and recent organ transplants who are over the age of 12 will be given a third jab following a recommendation from the Joint Committee on Vaccination and Immunisation (JCVI).

The announcement is separate to any decision on a booster programme, but it is understood news on that is expected soon.

Health Secretary Sajid Javid said the Government was continuing to plan for a booster programme to begin this month and said people most at-risk from Covid-19 would be prioritised for that.

The JCVI said it was still deliberating on the potential benefits of booster vaccines for the rest of the population and was awaiting further evidence to inform its decision.

No decision has been given either on any extension of the vaccine programme to include all healthy 12 to 15-year-olds.

Giving its recommendation on Wednesday, the JCVI said 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, meaning they could be less protected than the wider population.

Charities working with people who will be eligible for a third dose welcomed the announcement as it will offer “much-needed reassurance and additional protection”.

The committee said studies are ongoing to see how effective a third dose is for this group of people, and as it is considered unlikely to cause any harm, they have decided that a third jab can be safely offered and might boost protection.

The preference for children in this category who are aged 12 to 17 is the Pfizer vaccine, while those aged 18 and over will be given either Pfizer or Moderna.

Both of these jabs are mRNA vaccines and almost all of the data so far on third doses is based on this kind of vaccine.

The JCVI said the timing of a third dose will be made by a patient’s specialist doctor, but that they would usually be given at least eight weeks after the second dose, with some flexibility on that.

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, but is estimated to include between 400,000 and 500,000 people, or less than 1% of the population.

Mr Javid said he has accepted the JCVI’s recommendation.

He 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.

“The NHS will contact people as soon as possible to discuss their needs and arrange an appointment for a third dose where clinically appropriate.

“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. We will prioritise those most at risk to Covid-19, including those who are eligible for a third primary vaccine, for boosters based on the final advice of the JCVI.”

GPs and consultants will be involved in identifying eligible patients and delivering jabs, with more details to be set out.

Professor Jonathan Van Tam, deputy chief medical officer, said: “We know there are people with severe immunosuppression for whom the first two doses of vaccine have not provided the same level protection as for the general population. The degree of protection will vary by individual, according to degree of immunosuppression and the underlying reasons for that.

“So I welcome the advice from JCVI to offer a third primary dose to those with severe immunosuppression, at a bespoke interval, advised by their specialist clinician, and guided by the UK’s immunisation handbook, the Green Book.

“We should be doing all we reasonably can to ensure that this group is not disadvantaged and a third primary dose is one step in this direction. We are also working hard to ensure there are other medical interventions that can be used in these groups, including specific treatments like antivirals and monoclonal antibodies.”

People considered to have had severe immunosuppression at the time of vaccination include those with acute and chronic leukaemia, those on immunosuppressive therapy, those with chronic immune-mediated inflammatory disease and people who had received high-dose steroids for any reason in the month before their jab.