People who have already caught Covid at risk of Delta strain as it is four times more likely to beat antibodies

The findings help explain why the virus is spreading so rapidly across the country, particularly among young adults who are less likely to be double-jabbed.

Around 25,000 people are being infected with Covid every day in the UK at the moment.

But Boris Johnson is optimistic to go ahead with Freedom Day on July 19 because vaccines still work against the Delta strain to protect against hospitalisations and deaths.

Almost two-thirds of adults – 64 per cent – are now double-jabbed.

And one dose of the Pfizer vaccine is 36 per cent effective against symptomatic illness from the Delta variant, while one jab of AstraZeneca is about 30 per cent effective, according to Public Health England.

Although less is officially known about Moderna's performance against Delta, it uses the same technology to develop immunity as the Pfizer vaccine.

Experts in the UK are working on the assumption its effectiveness is largely the same as Pfizer, and works very well after two doses against the Delta variant.

The Delta variant now makes up almost every new Covid case in the UK - 95 per cent.

But it only arrived on British soil in mid-April, mostly in international travellers.

Its biological advantage of being able to spread faster than any other variant allowed it to become rampant.

Data from the Wellcome Sanger Institute reveals that in almost every local authority of England, the Delta variant is the most prominent strain.

In no more than six weeks, the Delta strain went from a handful of cases to making up the majority of infections in the UK.

Cases per week are still highest in areas of the North West, where the variant first hit.

Stephen Griffin, an associate professor in the School of Medicine, University of Leeds, said Delta is able to transmit "far higher, even with restrictions holding some in check, than the original strain".

Dr Peter English, a retired consultant in Communicable Disease Control, said: "Variants that can spread more effectively - like the Delta variant - have an advantage.

"This means that the virus can far more effectively seek out and find and infect people who are vulnerable.

"People who are partially vaccinated are less well protected against the now-prevalent Delta variant."

Experts are particularly concerned that new variants may come along and cause chaos in the UK.

Dr Julian Tang, an honorary associate professor and clinical virologist, University of Leicester, said: "Vaccine escape variants are more likely to arise in a partially immune population.

"Opening international travel again will likely import new variants that may start to replicate in the UK population - and may recombine with existing variants here."

However, he added: "But all of this is part of the process in 'learning to live with the virus' - and, as with flu, unfortunately, there will be casualties on the way."