Patients with conditions such as blood cancer and HIV may not have generated a good response after their first two jabs, it was said.
They will be offered an extra dose of either the Pfizer/BioNTech or Moderna jabs in the coming weeks.
The Joint Committee on Vaccination and Immunisation said it is hoped the extra inoculation will “top up” their response.
Health Secretary Sajid Javid said: “We know people with specific conditions that make them particularly vulnerable to Covid-19 may have received less protection against the virus from two vaccine doses.
“I am determined to ensure we are doing all we can to protect people in this group and a third dose will help deliver that.
“The NHS will contact people as soon as possible to discuss their needs and arrange an appointment for a third dose where clinically appropriate.”
A recent study showed that four in 10 people with impaired defences had a lower antibody response than would usually be expected after two doses.
Patients who will be eligible for a further shot include those taking drugs that suppress their immune systems, such as recent organ transplant recipients.
However, officials have stressed that the extra jabs were not part of the UK’s planned booster programme.
The JCVI issued interim advice in June outlining plans for a possible booster campaign covering vulnerable groups and all over-50s.
But despite the NHS preparing to start from Monday, a final decision has not been made.
It is understood the committee is awaiting further evidence. A decision on who should get boosters is expected “very soon”.
Mr Javid added: “This is not the start of the booster programme.
“We are continuing to plan for this to ensure the protection people have built from vaccines is maintained over time and ahead of the winter. We will prioritise those most at risk to Covid.” Family doctors and consultants will identify between 400,000 and 500,000 patients with impaired immune systems who are eligible.
Gemma Peters, chief executive at Blood Cancer UK, welcomed the decision but warned patients should still be careful even after a third dose.
She said: “People with blood cancer are at higher risk of becoming seriously ill from Covid and are less likely to have responded well to two doses of the vaccine, so it is great news that they will now be offered a third dose.”
A further 35,693 Covid-19 cases were confirmed across the UK yesterday and 207 deaths within 28 days of a positive test.
COMMENT BY DR. MATTHEW BUCKLAND
PEOPLE with immune systems that don’t function properly are among the most vulnerable to Covid-19, making providing them with protection a priority.
Yesterday’s announcement will provide reassurance, particularly for those who may not have responded optimally to their initial vaccine doses because of their clinical condition – for example, if they were receiving chemotherapy.
There is lots of variation in how those with impaired immune systems respond to jabs. Research found 40 percent have a lower response than healthy individuals.
The “third-dose” strategy aims to maximise the chances of increasing protection for this vulnerable group while minimising the risks they face.
However a small minority cannot generate an immune response after vaccination, such as those born without the required “machinery”.
It’s critical that we keep pursuing alternatives such as passive antibody therapies recently approved in the UK and awareness that these people need social distancing to stay safe.
- Dr. Matthew Buckland is the British Society for Immunology spokesperson