Both the Oxford and Pfizer jabs offer good protection against infection from the Delta variant of coronavirus – but those who do test positive can easily pass the infection on, a major new study suggests.

The research found that double-jabbed people who become infected with the strain have similar peak levels of virus in their nose and mouth than unvaccinated people – meaning they can easily transmit it.

Vaccines appeared to offer greater protection from infection against other variants, experts say.

Two doses of Pfizer appears to have greater effectiveness against the strain first identified in India than the Oxford vaccine – but its efficacy also declines faster, according to the preliminary findings.

Oxford University scientists regularly PCR tested more than 700,000 random people, who were analysed from before and after May 17, 2021, when Delta became the UK’s dominant variant.

Previous studies have often focussed on hospitalised people and come to promising conclusions about vaccine protection from death and severe illness.

Researchers found that after four to five months, the level of protection offered by both vaccines is similar, with the AstraZeneca jab maintaining its effectiveness throughout that period.

The findings, which have not yet been peer reviewed, suggested that for infections with a high viral load, protection a month after the second Pfizer dose was 90% greater than an unvaccinated individual, reducing to 85% after two months and 78% after three.

For AstraZeneca, the equivalent protection was 67%, 65% and 61%, the researchers said.

They added that although jabs did not eliminate chances of getting Covid-19, they did reduce the risk and remained the most effective way to ensure protection against the virus.

Sarah Walker, professor of medical statistics and epidemiology at Oxford University, 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.

‘This means it is essential for as many people as possible to get vaccinated – both in the UK and worldwide.’

The study, conducted in partnership with the Office of National Statistics (ONS) and the Department for Health and Social Care (DHSC), looked at data between December 2020 and August 2021 from the Covid-19 Infection Survey.

Dr Koen Pouwels, senior researcher at Oxford University’s Nuffield Department of Population Health, said that the team ‘can be confident’ that the numbers ‘really represent a decline’ for the Pfizer vaccine, whereas for Oxford/AstraZeneca ‘the differences are compatible with chance, that is there could be no change at all in the protection from AZ’.

‘Even with these slight declines in protection against all infections and infections with high viral burden, it’s important to note that overall effectiveness is still very high because we were starting at such a high level of protection’, he added.

‘It is also worth highlighting that these data here do not tell us about protection levels against severe disease and hospitalisation, which are two very important factors when looking at how well the vaccines are working.

But with previous research showing jabs are very effective against stopping hospitalisations, Dr Pouwels said: ‘The vaccines are better at preventing severe disease and are less effective at preventing transmission’.

The scientists also found that a single dose of the Moderna vaccine had similar or greater effectiveness against the Delta variant than a single doses of the other vaccines, but the researchers do not yet have any data on second doses of the US-made jab, which is also being used in the UK.

The study also suggested that the time between doses did not affect effectiveness in preventing new infections, and that younger people (aged 18-34) had more protection from vaccination than older age groups (35 to 64-year-olds).

The academics said they could not comment on what the study might mean for the potential autumn booster campaign, but said the Joint Committee on Vaccination and Immunisation (JCVI) would be considering the data while making its decision.

NHS officials have geared up to start offering a third coronavirus jab from early September, but are yet to be given the green light to press ahead from the JCVI.

Dr Alexander Edwards, associate professor in Biomedical Technology at the University of Reading, who was not involved in the study, said: ‘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.’

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