More than half of double-vaccinated blood cancer patients are left without effective protection against coronavirus, new research has found.
Data from a trial published in Cancer Cell shows 57% of blood cancer patients failed to develop the effective antibodies for an immune response to the virus after their second jab.
The study authors argued for "protective measures" such as mandatory mask-wearing and social distancing to continue, and for those affected to receive their additional jab, following the trial.
The trial, led by King's College London and the Francis Crick Institute, with support from Cancer Research UK, assessed 159 people at Guy's and St Thomas' NHS Foundation Trust, including 72 solid cancer and 56 blood cancer patients and their responses to a second dose of the Pfizer jab.
Just 36% of blood cancer patients were able to produce an effective immune response after their second jab.
The study also showed only 38% of solid cancer patients, such as those with breast, urological or skin cancers, were able to produce antibodies following single-dose vaccination but this grew to 78% following the second jab.
Lead author Dr Sheeba Irshad, a senior clinical lecturer from King's College London, said: "Covid-19 vaccines are very effective and safe for majority of the population, but people with moderate to severe compromise of their immune system are not completely protected after the initial dose or after both doses.
"And so, masks and other Covid-19 protective measures continue to remain necessary for patients particularly with blood cancers.
"It is also important for our patients to take up the offer of additional doses of Covid-19 vaccine recommended to them as part of the continued primary vaccination series."
Professor Charles Swanton, chief clinician at Cancer Research UK, said: "This study shows that over half of blood cancer patients are unable to mount an antibody response to SARS CoV2 despite being vaccinated twice, which we know is an important step in preventing severe infection risk.
"These results add to a growing body of evidence confirming the vulnerability of patients with blood cancers during the Covid pandemic despite being vaccinated.
"As the world begins to return to normal, we must not forget vulnerable patients like this, who will need ongoing measures to protect them from transmission and additional approaches to reduce the risk of severe disease.
"If not, we could see them being confined to isolation approaches for the foreseeable future."