Eight in 10 people with blood cancer have not been told they are not fully protected by the Covid-19 vaccine, a charity has warned.

An online survey by Blood Cancer UK of around 1,000 people found that 80% were not told by the NHS that their weakened immune systems lessened the chance they would have an immune response to the jab.

The charity said this means people with blood cancer are at a high risk of contracting coronavirus despite having both doses of a vaccine, and are also more likely to get severely ill.

Gemma Peters, chief executive of Blood Cancer UK, 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.

“So 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.”

Recent research suggested people with blood cancer account for an increasingly high proportion of Covid admissions to intensive care.

Some 230,000 people in the UK are living with blood cancer.

The charity is calling for the Government and the NHS to launch a campaign about vaccine efficacy in immunocompromised people and to urgently contact everyone in the UK to let them know what precautions they can take.

Ms Peters said: “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. Given the rising infection rate, failing to act quickly could result in more unnecessary deaths.”

A study in March found that three weeks after a first dose of the Pfizer vaccine, an antibody response was only seen in 13% of people with blood cancer, compared with 39% of people with solid cancers and 97% of people without cancer.

Further research by cancer charities including Blood Cancer UK looked at antibody response in 129 people with lymphoma.

Early results suggest 61% of participants who were vaccinated during or within six months of receiving systemic anti-lymphoma treatment did not have detectable antibodies despite two doses of vaccine.

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 still urging people with blood cancer to get both doses of a vaccine, but to also be aware they are not fully protected.

A spokesperson for the NHS said: “The NHS is vaccinating in line with clinical guidance set by the JCVI and clinicians should ensure their patients are aware of the latest advice – the current guidance is that people with blood cancer should continue to get both doses of the vaccine.”

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.”