An Oxford University study found the extra shot more than six months later pushed antibody levels above those measured after the second dose. Scientists said there was no evidence that boosters will be needed this autumn. Professor Sir Andrew Pollard, director of the Oxford Vaccine Group, said it would not be ethically right to give out more jabs while other countries are waiting for first doses.

       He said: “At this point with high levels of protection in the UK population and no evidence of that being lost, to give third doses now in the UK whilst other countries have zero doses is not acceptable.”

       In the study, 90 volunteers had a third dose six months after their second, which boosted their antibody and T-cell responses.

       Leaving a longer gap of up to 45 weeks between the first and second doses did not compromise the immune response. Prof Pollard said this would be reassuring for countries with low vaccine supplies.

       Another Oxford University study showed that doses of the AstraZeneca and Pfizer/BioNTech vaccines could be safely mixed.

       Giving one vaccine and then the other four weeks apart led to a higher antibody response than the threshold set by two doses of the AstraZeneca vaccine.