One shot of the Pfizer vaccine gives inadequate protection to cancer sufferers, new research suggests.
Scientists said the current strategy of delaying the second dose of the Pfizer vaccine must be urgently reviewed for cancer patients in light of the findings.
A study from King's College London and the Francis Crick Institute - which has not yet been peer-reviewed - found that three weeks after the first jab, antibody responses were found in 39 per cent of people with solid cancers and 13 per cent of people with blood cancer.
This compared with 97 per cent of people with no cancer, according to the research on 205 people, who comprised 151 with cancer and 54 healthy controls.
Cancer patients given a second dose of the vaccine three weeks after the first - as recommended by Pfizer - had a much better immune response, with 95 per cent of those with solid tumours showing detectable antibodies.
The team said leaving up to a 12-week gap between doses - as is currently happening in the UK - is leaving cancer patients vulnerable to serious Covid-19.
However, Cancer Research UK said the study was relatively small and people should continue to follow the advice of their doctors.
Dr Sheeba Irshad, senior clinical lecturer from King's College London, said: "Our data provides the first real-world evidence of immune efficacy following one dose of the Pfizer vaccine in immunocompromised patient populations.
"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.
"Until then, it is important that cancer patients continue to observe all public health measures in place such as social distancing and shielding when attending hospitals, even after vaccination."
Professor Adrian Hayday, from King's College London and the Francis Crick Institute, said: "The vaccine is very impressive in its impact on healthy individuals and our study shows that it can clearly bring immense benefit to cancer patients too, but in most cases this is only after boosting.
"Cancer patients should be vaccinated and boosted quickly and their responses, particularly those of blood cancer patients, should be intensively monitored so that those who mix with family, friends and carers can be confident of their environment."
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: "Worryingly, this study suggests that people affected by cancer, including breast cancer, get little protection against the virus when they only receive a single dose of the Pfizer Covid-19 vaccine and then do not receive their vaccine boost in the following three weeks.
"In contrast, the study identifies that when patients received a second dose of the vaccine within three weeks, they had significantly improved immune response and protection against coronavirus.
"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 spokesman 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."