They are calling for the government to urgently review its plan to delay the second dose of the vaccine.
However, other experts have cautioned that more tests need to be carried out first and cancer patients should follow the current advice.
A study from King's College London and the Francis Crick Institute, which has not yet been peer-reviewed, found that although 97% of people with no cancer had antibody responses three weeks after the first jab, the proportion for cancer patients was much lower.
For people with solid cancers, just 39% had antibody responses after the first dose. And for blood cancer patients, the figure was as low as 13%.
Cancer patients who had the second jab three weeks after the first, as recommended by Pfizer, had a much better immune response, with 95% of those with solid tumours showing antibodies.
The study looked at 205 people, 151 of who had cancer.
Researchers said the UK government's strategy of leaving a 12-week gap between both doses is leaving cancer patients at risk of coronavirus.
Dr Sheeba Irshad, senior clinical lecturer from King’s College London, said: “We show that following first dose, most solid and haematological (blood) cancer patients remained immunologically unprotected up until at least five weeks following primary injection; but this poor one-dose efficacy can be rescued with an early booster (second dose) at day 21.
“Based on our findings, we would recommend an urgent review of the vaccine strategy for clinically extremely vulnerable groups."
But Cancer Research UK said the study was relatively small and needs to be further scrutinised.
Martin Ledwick, head information nurse at Cancer Research UK, said: “This is an interesting study and it’s important to assess how cancer patients are responding to the vaccines being rolled out.
“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.”
Dr Simon Vincent, director of research, support and influencing at Breast Cancer Now, who supported the study, said: “In light of these findings we are calling on the Joint Committee on Vaccination and Immunisation to urgently review the evidence presented in this study, and to consider adapting its strategy to ensure that people who may benefit from this approach, including those with breast cancer, receive both the first and second dose of the Pfizer Covid-19 vaccine within a three-week timeframe to minimise their risk of both contracting and becoming seriously ill with coronavirus.”
A Department of Health and Social Care spokesperson said: “We are focused on saving lives and the antibody response is only part of the protection provided by the vaccine.
“The independent Joint Committee on Vaccination and Immunisation, which advises Government on vaccine use and prioritisation, regularly reviews data and evidence on vaccine efficacy and effectiveness.”
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