People who become infected with the Delta variant even after being fully vaccinated may still pass the virus on, undermining hopes of herd immunity, new research suggests.

Scientists at the University of Oxford found levels of the virus could be just as high in people who get COVID despite having both jabs as in those who haven’t been vaccinated.

Dr Koen Pouwels, one of the lead researchers of the study, said: “The vaccines are better at preventing severe disease and are less effective at preventing transmission.

“The fact that you see more viral load (with the Delta variant) hints towards herd immunity being more challenging.”

Scientists from the COVID-19 Infection Survey have conducted regular PCR tests on more than 700,000 randomly selected people since December last year.

Until mid-May, when the Alpha variant was the dominant form of the virus, the vaccines were highly effective at stopping infections.

But since then, a period when the more infectious Delta variant has dominated, the vaccines have been less able to block the virus.

Professor Sarah Walker, Chief Investigator for the Survey, said: “We don’t yet know how much transmission can happen from people who get COVID-19 after being vaccinated – for example, they may have high levels of virus for shorter periods of time.

“But the fact that they can have high levels of virus suggests that people who aren’t yet vaccinated may not be as protected from the Delta variant as we hoped.”

The researchers stress that real-world data shows that the vaccines are still highly effective at reducing hospital admission and death.

Because volunteers in the study had regular PCR swabs and antibody tests the scientists were able to monitor immunity over time.

They found that two weeks after the second dose the Pfizer jab was 85% effective at preventing PCR-confirmed infection, considerably better than the AstraZeneca shot, which was 68% effective.

But the protection of the Pfizer jab waned more rapidly. Three months after the second dose it prevented 75% of infections, compared to 61% protection with the AstraZeneca jab.

“These two vaccines look like they are working in very different ways,” said Professor Walker.

“But it’s the rates of hospitalisations and deaths that we really need to keep our eye on.”

The study has been published as a pre-print and has not been peer-reviewed.

Dr Alexander Edwards, Associate Professor in Biomedical Technology at the University of Reading, said the results were “compelling”.

“Overall this study is excellent as it shows that although Delta is better at infecting vaccinated people than previous variants, the vaccines still work remarkably well.

“There are subtle differences between different vaccine types, and some changes over time, but they all work brilliantly.

“It does remain vital to remember that even if double jabbed, you can still get infected and pass the virus on.”

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Prof Paul Hunter, Professor in Medicine at the University of East Anglia, said: “There is now quite a lot of evidence that all vaccines are much better at reducing the risk of severe disease than they are at reducing the risk from infection.

“The main value of immunisation is in reducing the risk of severe disease and death and the evidence available shows that protection lasts longer against severe disease than against mild disease and all current UK vaccines are very good at this even against the Delta variant.”