One in ten patients with weakened immune systems failed to produce antibodies after two Covid vaccinations - but the vast majority remained protected against severe disease.
The landmark Octave trial, led by Glasgow University, followed 2,686 immuno-compromised patients up to one year after their first vaccination, including 2,204 individuals who had received two doses by mid-2022.
It found that 255 participants (12%) had failed to generate any Covid antibodies after two doses, while an additional 600 (27%) had only low levels.
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Vaccine failure rates were highest, at 72%, among patients with ANCA-associated vasculitis - a group of autoimmune disorders characterised by inflammation and damage to small blood vessels - who were also taking the drug rituximab.
There were also high vaccine failure rates among organ transplant patients (31%) and kidney failure patients who were receiving dialysis and also using immuno-suppressing drug therapies (20%).
Data is not yet available for responses triggered by a third or fourth vaccine dose.
Antibodies are the immune system's first-line defence to ward off infection by preventing the virus from entering cells.
However, vaccines are also designed to elicit a cellular response in the form of T cells, which can attack the virus once it is inside cells to reduce disease severity.
A T cell response was detected in 513 of the 580 (88%) patients tested.
Patients who received the Pfizer-BioNTech Covid vaccine were more likely to mount an antibody response, but the Oxford-AstraZeneca vaccine generated a stronger T cell response.
The UK-wide Octave trial was set up in the middle of the pandemic to evaluate immune responses following Covid-19 vaccination in transplant and clinically vulnerable patients with conditions such as cancer, inflammatory arthritis, and diseases of the kidney or liver.
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Preliminary data in August 2021 indicated that a significant proportion mounted a low, or undetectable, immune response after two vaccine doses, but peer-reviewed data published in the journal Nature Medicine reveals for the first time details of infections, hospitalisations, and mortality.
In total, 474 patients in the study tested positive for Covid in the year following their first vaccination.
Of the 440 participants where case severity was known, 348 (79%) had mild infections - meaning they did not require hospitalisation - and 49 (11%) were asymptomatic.
Of the remaining 43, 33 were hospitalised and 10 died.
Deaths were reported in patients with kidney failure or who had received kidney transplants, who had ANCA-associated vasculitis, or who had undergone treatments which strip back the immune system such as CAR-T for cancer or stem cell transplants.
Low rates of severe disease were reported in patients with Crohn’s disease and ulcerative colitis.
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Most infections (76%) occurred more than six months after the second vaccination, and patients infected during the Delta wave were more likely to experience serious symptoms than those infected during 2022 when Omicron was dominant.
Study lead Professor Iain McInnes, who heads Glasgow University's College of Medical, Veterinary and Life Sciences, said the findings were reassuring.
He said: "The study’s key strengths include identifying the small number of patients who may not respond to the vaccines, enabling healthcare providers and policy makers to make the best decisions to protect these groups of people.
"Importantly the study has also been able to reassure us that the majority of our immunocompromised patients in the UK have been protected from severe Covid-19 by the vaccination programme.”
Professor Andrew Ustianowski, clinical lead for the Covid-19 vaccination programme at the National Institute for Health and Care Research (NIHR), said the study had been "pivotal" in identifying patient sub-groups at highest risk.
Professor Pamela Kearns, director of the Cancer Research UK Clinical Trials Unit at the Birmingham University - which coordinated the trial - said: “We can see that there are areas of particular concern where vaccines didn’t adequately protect against Covid 19, including some patients with renal diseases and some inflammatory conditions."
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