The Covid reproduction ‘R’ number has risen to 1.4, but vaccinating younger people should slow down the exponential rise in cases, scientists believe.

Researchers at Imperial College have been carrying out regular prevalence swab testing on up to 180,000 people in England each month for the React-1 study since the beginning of the pandemic.

The latest results show there has been a 50 per cent increase in infections in the two weeks up to June 7 compared to May 3.

The current government estimate suggests the ‘R’ number is between 1.2 and 1.4, which puts the Imperial figure at the higher end and means every 10 people would go on to infect a further 14. 

But the React-1 team discovered there was a five-fold difference in the risk of infection between younger and older groups, suggesting vaccinations are still protecting the most vulnerable.

Although they warned that there was now exponential growth in infections – with cases doubling roughly every 11 days – they believe rolling out the vaccines to younger groups should stop the rise.

How many people have been vaccinated?

Steven Riley, professor of infectious disease dynamics at Imperial, said: "Prevalence is increasing exponentially, and it’s being driven by younger ages. Clearly that is bad news, but the key thing to point out here is that we’re now in a very different part of the epidemic – we know there is a lot of immunity in the population.

"The portion of population that is driving the growth is all the proportion that’s not vaccinated. Rapid rollout and mass vaccination of the younger ages, which is where we see the bulk of the infections, should effectively slow that growth."

The team mapped the number of infections to hospitilisations and deaths with a lag of 19 days between infection and admissions, and 26 days between infection and death. 

They found that at the beginning of the year there had been a clear link between the level of infections and deaths, but that had started to wane from around March with a clear fall in the number of deaths per case. "Each unit of prevalence is producing fewer and fewer deaths," said Prof Riley. 

However, although the trend had continued in the over-65s, the number of cases leading to deaths had started to rise again for younger people not protected by the vaccine. 

The results also showed that the percentage of people testing positive was highly dependent on being vaccinated. Just 0.6 per cent of fully vaccinated over-65s were found to have an infection, compared to 0.23 per cent of the unvaccinated under-65s.

Prof Paul Elliott, chair in epidemiology and public health medicine at Imperial, said: "I think we take quite a lot of comfort when we look at the details. It does appear there is very good protection where everyone is double vaccinated. 

"There is a delay between vaccines going into the arm and protection, but the sooner we get vaccines in the arms the sooner we’ll get protection."

On Monday, the Government announced a delay to the roadmap to allow more people to be vaccinated, with an extra 10 million people expected to receive their second jab by July 19.

How many more could we vaccinated with the delay to June 21?

Commenting on the latest data, James Naismith, director of the Rosalind Franklin Institute at the  University of Oxford, said: "The case for increasing the coverage of the double vaccinated as rapidly as possible is clear.  

"I am confident that when and if we reach 85 per cent  coverage of double vaccination of the population capable of onward spread, then the UK will indeed be able to live with the virus."

Experts said the rates are still "very low" compared to previous waves, with current prevalence rates similar to August. 

Prof Kevin McConway, emeritus professor of applied statistics at the Open University, said it was clear the vaccines were offering protection, meaning the country was in a different place to when cases have risen before.

"There’s certainly protection," he said. "If that’s the case, we don’t have to be as concerned with increasing cases as we were the last time they were at this kind of level, last September. 

"We do still have to be careful and keep looking at the data, because there’s still a lot of uncertainty about some of the details. But there’s more hope."