A UK trial is being extended to test the effectiveness of a third Covid jab in people who are immunocompromised or immunosuppressed.

It follows preliminary data from the OCTAVE study that 40% of those with weakened immune systems mount a low or undetectable response after the standard two doses.

The same researchers have now been awarded £2.2 million to test a third dose of either Pfizer, Moderna or Novavax vaccines in up to 1,200 participants already enrolled in the study, with initial results expected later this year.

It comes as researchers behind the ZOE Covid Study app say they are seeing some signs of waning protection in people who have been double jabbed.

Lead investigator Professor Tim Spector said their data suggests that protection after two Pfizer vaccines fell from 88% at one month to 74% at five to six months with a fall of 77% to 67% for AstraZeneca vaccination over the same timescale.

He added: ‘In my opinion, a reasonable worst-case scenario could see protection below 50 percent for the elderly and healthcare workers by winter.’

Results from the first phase of the OCTAVE study found that 89% of people who are immunocompromised or immunosuppressed generate antibodies following vaccination, and 60% generated a strong antibody response after two doses.

Participants included people with lymphoid malignancies, immune mediated inflammatory diseases (including rheumatoid arthritis, psoriatic arthritis, vasculitis and inflammatory bowel disease), renal disease, solid tumours, hepatic and intestinal disease, primary immune deficiency and people who had undergone haematopoietic stem-cell transplantation.

Researchers pointed out that while antibody levels after vaccination varied, it is still not known how this corresponds to protection from Covid-19 with T-cells also likely to have an important role

The Government will now fund the OCTAVE DUO study which will analyse the immune response and the durability of protection after a third dose but also rates of Covid-19 infection.

Pharmacies in England are preparing for a booster programme for the most vulnerable due to run from sometime in September, although exactly who will be prioritised is yet to be finalised.

A final decision from the Joint Committee on Vaccination and Immunisation on exactly who will receive a booster this winter has yet to be announced.

The Government said it would carefully consider results from the OCTAVE trial and advice from the JCVI on whether those who are immunocompromised or immunosuppressed should be offered an additional vaccine dose

Health and social care secretary Sajid Javid said: ‘Vaccines have built a strong wall of defence in the UK and this is allowing most of us to learn to live safely with Covid-19.

‘We know some people may get less protection from the vaccine than others, so we are planning for a booster programme in the Autumn, prioritising those most at risk.

‘This new study will play an important role in helping to shape the deployment of future vaccines doses for these specific at-risk groups.’

Professor Iain McInnes, head of the College of Medical, Veterinary and Life Sciences at the University of Glasgow and lead of the OCTAVE and OCTAVE DUO studies, said their results had shown that there is a group of patients who may not mount a sufficient immune response.

‘We are pleased to now roll-out of the OCTAVE DUO trial, to investigate the effects of a third dose on this particular group of patients who have shown an undetectable or low vaccine response. We hope to provide answers to this very important unanswered question.’

This week, the UK Government announced an order of a further 35 million Pfizer/BioNTech Covid vaccines due to arrive from the second half of next year, which could be used for potential future booster programmes.

A version of this story first appeared on our sister title, Pulse.