Half a million people in the UK who have severely weakened immune systems are to be offered third shots of Covid vaccines in an effort to improve their protection against the disease.

So what is the advice, who is eligible, and where does this leave plans for an autumn booster programme?

What is the new advice on vaccines?

The government’s independent vaccine advisers, the Joint Committee on Vaccination and Immunisation (JCVI), has recommended a third shot of Covid vaccine for people with severely weakened immune systems. The committee estimates 400,000 to 500,000 patients aged 12 and over are eligible in the UK.

Are these boosters?

The JCVI stresses that these are not booster jabs, but third shots to be given as part of the primary vaccination schedule for less than 1% of the UK population. The committee is discussing whether to press ahead with an autumn booster programme as a separate issue.

In June, the advisers issued interim advice urging the NHS to prepare for a two-stage booster programme starting as early as September, with a final decision due in the summer. So far no decision has been made, though the committee says it will have more data to inform its recommendation very soon.

What prompted the new advice?

The recommendation for a third dose follows a number of studies into the effectiveness of the standard two shots in people whose immune systems are badly weakened either by a medical condition, such as HIV or leukaemia, or therapies that intentionally suppress immunity, such as drugs given to prevent people rejecting transplanted organs.

The recent Octave study, led by the University of Glasgow, found an estimated 40% of people with weakened immune systems had poor immune responses to Covid vaccination, raising concerns that many could remain vulnerable to severe disease even after two doses.

How will a third dose help?

A small number of studies have shown that third doses of Covid vaccine may help to improve protection in those with suppressed immune systems.

Under the JCVI advice, third shots will be given in the UK at a time decided by the patient’s GP or consultant to maximise the person’s chances of mounting a good immune response. This could be as straightforward as giving the dose before a person has immune system-suppressing chemotherapy rather than afterwards.

Which vaccines will be used?

Much of the data used to inform the JCVI’s decision comes from studies of mRNA vaccines, namely the Pfizer/BioNTech and Moderna jabs. These have both been shown to produce robust immune responses in healthy people and have good safety records.

With this in mind, the JCVI recommends that eligible patients aged 12 to 17 receive the Pfizer vaccine as their third shot, with those aged 18 and over receiving the same or the NIH/Moderna vaccine.

Who is eligible?

The JCVI has drawn up a comprehensive list of patients who may benefit. Among them are people aged 12 and over who have leukaemia, aggressive lymphomas, immunosuppression due to HIV/Aids; those who have had recent stem cell transplants, those who are having or had recent immunosuppressive therapies, and others on high doses of steroids.

Where does this leave an autumn booster programme?

The JCVI has yet to reach a final decision on a UK booster programme. While several studies suggest antibody levels wane in the months after vaccination, this is to be expected, and other parts of the immune system such as T cells and memory B cells may still provide good defences.

Scientists are watching closely to see whether waning antibody levels translate into more severe disease in those vaccinated early in the year. A major concern is that vaccine doses to be used in the booster programme are needed to prevent more deaths in other countries that have yet to vaccinate their most vulnerable.