People who have had both doses of the AstraZeneca vaccine could be three times as likely to develop Covid symptoms than those with Pfizer and Moderna, a new study has claimed.
Scientists at Imperial College London (ICL) reporting to SAGE estimate someone double-jabbed with the Oxford University-developed AZ jab - now called Vaxzevria - is 55 per cent less likely to get symptoms of the Delta variant.
While Pfizer and Moderna efficacy is thought to be close to 85 per cent, the experts add.
Using these percentages, for every 100 unvaccinated people who developed symptomatic Covid, 45 are expected to from two doses of AstraZeneca's jab.
While just 15 people, three times less, are expected to develop symoptoms for Pfizer or Moderna, according to the Imperial team.
However, a spokesman for ICL said there is uncertainty around such estimates, as not enough is known about the impact of the Delta variant, first identified in India last year.
He said age and health of each infected person creates too many variables to make the study straight forward.
The Delta variant continues to account for approximately 99 per cent of confirmed cases of coronavirus across the UK.
The scientists based their estimates on clinical trial data, including from Public Health England.
It comes after researchers suggested yesterday elderly people jabbed with AZ are less likely to develop antibodies than if they got Pfizer.
However, some such estimates imply Pfizer and Moderna are only actually marginally more effective than the Oxford alternative.
In the UK, around 24.6million people have had the AstraZeneca vaccine, while 19.1million have had Pfizer and one million Moderna.
Scientists at London School of Hygiene and Tropical Medicine, which also presents data to the SAGE panel, estimate 29 per cent of AZ recipients get virus symptoms.
That compares to 16 per cent with Pfizer or Moderna jabs.
While the University of Warwick says it's more like 18 percent for Oxford recipients and 17 for other jabs.
Pfizer and Moderna developed mRNA jabs which use genetic code to prompt an immune response against a virus.
The AZ alternative is a viral vector vaccine which contains a modified version of the common cold virus to trigger an immune response.
The three universities were in agreement that having two Oxford jabs slashed the odds of hospitalisation by 85 to 90 percent.
The mRNA jabs, meanwhile, are 89 to 91 per cent safer from being admitted, it is estimated.
And all three reduce the chance of death by 96 per cent, the SAGE report claims.
The Joint Committee on Vaccination and Immunisation (JCVI) ruled in May anyone under 40 should be offered an alternative to AZ if possible due to concerns about rare blood clots.
The UK's impressive Covid vaccine drive has seen almost everyone eligible immunised, with 35,341,428 people having had both doses as of Wednesday.
Professor Neil Mabbott, of the University of Edinburgh, told the Daily Mail the Imperial's modelling is pessimistic, with it "not quite as straight forward" as single percentages, as age and health of each infected person creates too many variables.
