People with blood cancer are unaware they might not be fully protected by the COVID-19 vaccine, a charity has warned.
Blood Cancer UK surveyed around 1,000 people and found that 80%, or four out of five, were not told by the NHS that their weakened immune systems lessened the chance of having an effective response to the coronavirus jab.
This means people with blood cancer are at a higher risk of contracting the virus, despite receiving two jabs and are more likely to become severely ill, the charity said.
"We have known since the start of the vaccination programme that immunocompromised people were less likely to be protected by the vaccines, and over the last few months research we have funded has shown that many blood cancers and treatments have a significant impact on vaccine response," said Gemma Peters, chief executive of Blood Cancer UK.
"I am really worried that many people with blood cancer have not yet been told this, and so cannot take informed decisions to better protect themselves even after their second jab."
The charity's research suggests blood cancer patients are accounting for a high proportion of COVID-19 admissions in intensive care.
Some 230,000 people in the UK are living with blood cancer.
The charity is calling on the government and the NHS to launch a campaign about vaccine efficacy in immunocompromised people.
"The government needs to communicate with every immunocompromised person to tell them they are at risk and to make this lifesaving message a key part of its communications," Ms Peters said.
"Given the rising infection rate, failing to act quickly could result in more unnecessary deaths."
A study in March found that three weeks after the first dose of the Pfizer jab, the antibody response was only seen in 13% of people with blood cancer, compared with 39% of people with solid cancers and 97% without cancer.
Professor Adele Fielding, president of the British Society for Haematology, said: "Vaccines are often less effective in patients with blood cancers, due to both their disease or their treatment. The COVID vaccines are no exception.
"They may offer much more limited protection to some patients with blood cancers and it is important that there is widespread awareness of this among patients, their families and their healthcare teams."
Blood Cancer UK is urging people to get both doses of the vaccine but to also be aware they are not fully protected.
A Department of Health and Social Care spokesperson said: "The antibody response is only part of the protection provided by the vaccine and we remain committed to ensuring as many people as possible are protected from the virus and continue to safely receive treatment.
"For example, our new antivirals taskforce is working to identify effective treatments for patients who have been exposed to the virus to stop the infection spreading and speed up recovery time - including those who are immunocompromised."