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

       People with conditions such as leukaemia, advanced HIV and recent organ transplants will be eligible for an extra jab following a recommendation from the Joint Committee on Vaccination and Immunisation (JCVI).

       Such groups will be offered an extra dose because research has found that two jabs give an inadequate immune response.

       However, health officials stressed that the announcement is “not the start of the booster programme” for a wider population.

       In interim advice back in June, the JCVI drew up plans for third jabs for all over 50s, with the NHS asked to prepare to be able to start such a rollout from September 6.

       But the committee has yet to issue final advice on the matter.

       Health Secretary Sajid Javid said the Government was continuing to plan for a booster programme to begin later this month, based on final advice from the JCVI.

       The committee 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 healthy 12 to 15-year-olds.

       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, leaving them less protected than the wider population.

       Last week UK trials found 40 per cent of people with impaired immune systems have “low or undetectable” antibodies after two Covid jabs.

       The committee said studies are ongoing to see how effective a third dose is for this group of people. 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.

       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."

       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."