The UK's health minister said on Wednesday that the government will offer a third vaccine jab to half a million people with severely weakened immune systems.
Health minister Sajid Javid said that a third dose will be offered by the state-run health service to those who "may have received less protection against the virus from two vaccine doses".
Javid announced the rollout after the government advisory body the Joint Committee on Vaccination and Immunisation (JCVI) recommended a third dose for those aged over 12 who have conditions such as leukaemia, advanced HIV or recent organ transplants.
It said that the third dose by preference should be an mRNA vaccine -- so, not the AstraZeneca jab.
The announcement comes as the UK government is considering whether to follow several other countries in issuing booster jabs to the population as a whole, potentially in early September alongside the flu jab.
Javid said that the "third primary vaccine" jab was not the same as a booster shot.
He said the government was "continuing to plan" for a booster programme to begin in September, prioritising those most at risk, including those eligible for a third vaccine dose.
The World Health Organization has condemned the rush by wealthy countries to provide Covid vaccine booster shots, while billions around the world have yet to receive a single dose.
In a report published on Wednesday, the European Centre for Disease Prevention and Control said there was no urgent need to administer booster jabs to the fully vaccinated -- though it said an extra dose may benefit those with weak immune systems.