Research by King's College London and the Francis Crick Institute found a second dose boosted protection for people with cancer, but that waiting 12 weeks may leave them at a higher risk of infection.
The first real-world study in this area of medicine looked at 205 people suffering from the disease, including 151 with solid cancers, such as breast and bowel, and others with blood cancers.
Volunteers were tested for antibodies and T-cells in their blood – signals that an immune response has been triggered, suggesting they have some level of protection against illness.
Scientists found that three weeks after the first dose, 39 per cent of people with solid cancers and 13 per cent of people with blood cancer had an antibody response.
That was significantly lower than non-cancer patients, where protection extended to 97 per cent of people.
A second dose three weeks after the first, which is recommended by Pfizer and which some of the cancer patients received, increased the protection level of that cohort to 95 per cent.
After five weeks in the group with only a single jab administered, there was little if any increase in protection for cancer patients, with 43 per cent of people with solid cancers and 8 per cent of people with blood cancer showing an antibody response.
For those without cancer, 100 per cent of people were offered some level of protection.
Dr Sheeba Irshad, an oncologist and senior study author from King's College London, recommended the government review the spacing of vaccine doses in light of the new study, which is yet to be peer-reviewed.
“Cancer treatments have profound effects on the immune system and cancer patients' immune mechanisms are inferior,” she said.
”We need to be concerned about other vaccines for this population too – they do need a second dose quickly.”
However Martin Ledwick, head information nurse at Cancer Research UK, urged caution over the findings which relied on a “relatively small” number of trial participants.
“This is an interesting study and it’s important to assess how cancer patients are responding to the vaccines being rolled out,” he said.
“But at this stage, we are looking at data that hasn’t been peer-reviewed, where other experts in the field would flag errors and limitations within the results.
“The numbers of patients looked at in the study are also relatively small, particularly for those with blood cancers.
“We know that this information could be worrying, but anyone undergoing cancer treatment should continue to follow the advice of their doctors, and we encourage all who can to take up the vaccine.”
Gemma Peters, chief executive of Blood Cancer UK, said: “We have been concerned about how much protection the vaccines will give people with blood cancer because vaccines do not usually work as well for people with compromised immune systems.
“This study, while not peer reviewed and only looking at a small number of people, is worrying news in that it adds to that concern.
“This means that if you have blood cancer, it is important that you do not assume you have protection even after you have had the vaccine, particularly after just one dose, and that you continue being careful to avoid Covid.
“But while this news is concerning, people with blood cancer should still definitely have the vaccine, as it is safe and even a smaller chance of protection is much better than none.”
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