Experts said that the Cov-Boost study, which was testing booster shots, showed that T-cell and antibodies were very good, but a third dose had not yet been measured against clinical disease. 

However government scientists said they had “every reason to believe” it would lead to good effectiveness. 

Winter Covid plan 

On Tuesday afternoon, the Government will set out its blueprint for ‘living with the virus’ through the winter, with a third jab on offer for all over-50s as part of the package.

Ministers believe it will help ensure the NHS is not overwhelmed by new cases as it moves into autumn and winter.

Sajid Javid, the Health Secretary, will also set out the details when he unveils the Government’s winter Covid plan for England in a Commons statement.

Nadhim Zahawi, the vaccines minister, said further lockdowns would be an “absolutely last resort” under the plan.

“We’re about to embark on a massive booster campaign and of course a flu vaccination programme – I am concerned about flu, we haven’t had much flu circulating anywhere in the world, and in a bad year we could lose up to 25,000 people to flu,” he told Sky News.

“The direction we’re taking – a massive booster campaign, a varied surveillance system, a really fit for purpose test and trace system – that is where we think we will be able to end up, confident that we have the infrastructure in place to deliver on this.

“But of course, we have to have contingency planning. All the time we have information coming in on how the virus is behaving.

“Winter gives the virus an inbuilt advantage – boosters reduce that advantage by hopefully taking the most vulnerable out of harm’s way.”

He told the BBC the boosters could be the “last piece of the jigsaw to allow us to transition this virus from pandemic to endemic” and by next year there could be a flu-style annual jab.