Coronavirus cases are "rising exponentially" among unvaccinated groups in England, according to scientists tracking the epidemic.
The study, commissioned by the government, found that rising cases are also being driven by young people.
Between May 3 and June 7 infections doubled, which coincides with the rise of the Delta coronavirus variant which was first detected in India and is now dominant in the UK.
Data from nearly 110,000 swab tests carried out across England between May 20 and June 7 suggests Covid-19 cases are doubling every 11 days, with the highest prevalence in the North West and on average one in 670 people are infected.
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The experts from Imperial College London said their findings show a “rapid switch” between the Alpha (Kent) variant, which first appeared in the UK in September 2020, and the Delta variant in the last few weeks, with the latter accounting for up to 90% of all coronavirus cases.
Earlier this week the government said the Delta variant now accounts for 96% of all Covid cases in the UK.
But they stressed that the country is in a much different position than autumn last year when an exponential growth triggered a second wave of coronavirus infections.
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He added: “And it appears to be doubling every 11 days.
“Clearly that is bad news… but the key thing to point out here is that we are in a very different part of the epidemic in the UK and it is very difficult to predict the duration of the exponential phase.
“Last autumn and last spring – when we were when we started to observe exponential growth – in some ways it was much more alarming because we knew there was very, very little immunity in the population.
“(But now) because of the vaccination (programme), we know there is a lot of immunity in the population.
“Even though there is a lot of immunity, it does not prevent some exponential growth and that is what we are seeing here.”
The scientists said their findings from the React study suggest that imminent expansion of the vaccine programme to those aged 18 and above “should help substantially to reduce the overall growth of the epidemic”.
Study author Paul Elliott, director of the React programme and chair in epidemiology and public health medicine at Imperial, said: “I think we can take quite a lot of comfort from the fact that when we look in the details, it does appear that there is very, very good protection in the older ages, where there is virtually everyone double vaccinated.
“And in the younger group under the age of 65, where a much smaller proportion have been vaccinated or double vaccinated, most infections are occurring in the unvaccinated group.
“And the government has clearly announced that they want to vaccinate all adults in the period between now and July 19, I think that will make a very big difference and increase the total amount of population immunity.”
The research, which has been published as a pre-print on an online server, shows the bulk of infections is being driven by children aged between five and 12, as well as younger adults aged between 18 and 24.
Infections in these age groups around five times higher when compared to those over 65, the researchers said.
Data showed that the “weakened link” between infection rates and hospital admissions was “well maintained” for those aged 65 and above, while “the trends converged below the age of 65 years”.
Prof Riley said: “We have observed this reconvergence in the pattern of hospitalisations and deaths versus infections, especially in an age group under 65.
“These patterns are consistent with two vaccine doses being highly effective.”
He added: “We expect the rapid rollout into those ages, especially first doses for people who have not had any dose, and then secondary says for those who’ve had one dose, should effectively slow that growth, even against all the other factors.”
Health and Social Care Secretary Matt Hancock said: “These findings highlight the stark context in which we took the difficult decision to delay Step 4 of the road map out of lockdown.
“Cases are now rising, but thanks to our incredible vaccination programme and enhanced response package including surge testing, we have the tools to curb the spread of this virus.
“We all must hold our nerve that little bit longer as our vaccine rollout continues and I urge everyone to keep observing hands, face, space and fresh air, and make sure you receive both doses of the vaccine for the best possible protection.”