Infection rates are three times lower among fully-vaccinated Britons, a major study has found.
Data from the REACT study — which randomly tested 100,000 people between June 24 and July 12 — found 0.4 per cent of double-jabbed adults tested positive.
But prevalence of the virus was 1.21 per cent among those who had not yet received either jab.
The findings mirror tests results from a month earlier, highlighting how the vaccines have really helped in slashing infections.
But the same study, which ministers rely on to track the outbreak, found two doses were less effective at preventing infection than previously thought.
Imperial College London academics warned it is 'highly likely' the country will face another wave in the autumn.
The Government-backed team suggested children should be vaccinated to prevent another resurgence of the virus.
It came after Nicola Sturgeon yesterday revealed all over-16s in the UK are set to be given approval for Covid vaccines within the next few days.
Any move to green light jabs for children would mark a huge U-turn by health chiefs, who have until now recommended against routinely inoculating youngsters.
Data from the study also revealed that two doses of a vaccine are 49 per cent effective at preventing asymptomatic infection, a marked decline compared to other estimates. But the protection offered by the vaccines rises to 59 per cent against symptomatic cases, researchers said
Cases are continuing to fall in the UK, with just 21,691 positive Covid tests recorded yesterday — with the figure being the lowest for five weeks.
The Imperial study, funded by the Department of Health, warned the recent decline in cases could reverse when schools reopen in September.
It concluded vaccinating more children could 'substantially reduce transmission' and have 'knock on benefits across the whole population'.
Researchers wrote: 'In our data, the highest prevalence of infection was among 12 to 24 year olds.'
They claimed it raises the prospect 'vaccinating more of this group by extending the UK programme to those aged 12-17 could substantially reduce transmission potential in the autumn when levels of social mixing increase'.
Half of all infections were in those aged five to 24 despite them only making up one in four of the population, reflecting lower vaccination rates in this age group.
Professor Paul Elliott, who led the study, said there was 'the opportunity to get more vaccine into people' over the summer.
Unveiling the findings in a briefing for health and science journalists, he said: 'Clearly the issue of vaccinating teenagers is very important and topical.
'It's under constant review by the Joint Committee on Vaccination and Immunisation (JCVI).'
He added: 'That age group is a potential source of infections, we've seen it in current data.'
Top graph: Data from June 24 to July 12 (orange bars), gathered by Imperial researchers as part of the REACT study, shows that infection rates were highest in five to 24-year-olds. Half of all Covid infections were in this group, despite them making up just 25 per cent of the population. Nine times more children aged 13 to 17 tested positive in the most recent testing window compared to rates from May 20 to June 7 (grey bars). Bottom graph: Figures also show that infection rates were highest in London, where 0.94 per cent tested positive by July 12, up from just 0.13 per cent in the previous study period
Positive PCR test samples taken as part of the REACT study closely follow the waves of the pandemic
