A third dose of the Covid-19 vaccine will be offered to everyone over the age of 12 years old with a severely weakened immune system, following advice from the Joint Committee on Vaccination and Immunisation (JCVI).

Around half a million people (less than 1% of the population) will be eligible for the jab and will be contacted by the NHS to discuss their needs and arrange an appointment where appropriate.

This subgroup of people does not constitute all those on the original list of clinically vulnerable people, but are rather people with severely suppressed immune systems, such as those with advanced HIV or blood cancer.

The announcement follows research showing that around 40% of people with weakened immune systems had a low antibody response, and therefore two doses provided less protection from the virus.

"This is not the start of the booster programme"

The government emphasise that this is not a booster jab (which is designed to extend already strong protection from the virus), but is instead for those who were likely not adequately protected from just two shots and are more likely to become seriously ill from Covid-19.

Health and Social Care Secretary Sajid Javid said: “This is not the start of the booster programme – we are continuing to plan for this to begin in September 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-19, including those who are eligible for a third primary vaccine, for boosters based on the final advice of the JCVI.”

The AstraZeneca vaccine will not be recommended for the third dose

Professor Jonathan Van-Tam, Deputy Chief Medical Officer, said he welcomes the advice from the JCVI to offer a third primary dose to those with severe immunosuppression.

“We should be doing all we reasonably can to ensure that this group is not disadvantaged and a third primary dose is one step in this direction. We are also working hard to ensure there are other medical interventions that can be used in these groups, including specific treatments like antivirals and monoclonal antibodies,” he said.

Currently, the JCVI is not recommending the AstraZeneca vaccine for the third dose because all available evidence is based on mRNA vaccines, like Pfizer.