England is on the brink of an unprecedented public health experiment, as it prepares to remove almost all Covid-19 restrictions while cases are increasing exponentially.

Sajid Javid, health secretary, told the House of Commons on Tuesday that the “protective wall” of vaccination made it possible to end social-distancing restrictions and “the health, social and economic hardship we know they bring”.

Although some scientists applauded the move to the final step of the unlocking road map on July 19, which prime minister Boris Johnson announced on Monday, others expressed varying degrees of caution or alarm.

“The wall of protection offered by vaccination is not solid — it’s affected by rising case numbers, hospitalisations and the ever present threat of new variants popping up that are vaccine-resistant,” said Professor Lawrence Young, a virologist at Warwick Medical School. “This a big gamble.”

Calum Semple, professor of medicine at Liverpool university and a member of the government’s Sage science advisory group, was more sympathetic. “I wouldn’t say this is a gamble, it’s more of a calculated risk,” he said.

Virologists are worried by the way vaccines are being used to “prevent illness in people rather than the usual primary aim of a vaccine, which is to prevent infection in the first place”, said Professor Richard Tedder of Imperial College London.

“Using these vaccines in the present way to free up our behaviour comes with the very real risk of facilitating the escape of variants which will be even more resistant to vaccines and potentially more infectious,” he said. “Failing to recognise this is playing with fire.”

Azra Ghani, professor of infectious disease epidemiology at Imperial, agreed. “By letting the virus run through the population, we are creating the perfect condition for the selection of mutations that allow the virus to evade the vaccine,” she said. “This strategy may therefore not only be risky for England but could also set back the global fight against the pandemic.”

The outlook is less clear than at any time in the past 15 months. “There’s been uncertainty throughout the pandemic but we’re now in a situation where it’s not just overall transmission that’s changing, it’s also the underlying relationship between the different data streams, such as infections, cases and hospitalisations,” said Adam Kucharski, associate professor in infectious disease epidemiology at the London School of Hygiene and Tropical Medicine.

The three main university groups that provide modelling advice to the UK government are updating their data this week. Their last projections, released in mid-June, showed a wide range of outcomes. This depended on the effectiveness of vaccines against Delta and other new variants; the durability of immunity; how transmissible the virus becomes; and the extent to which people maintain precautions such as mask-wearing and social distancing after legal restrictions end.

Chart showing that Covid hospital admissions in England are accelerating upwards, and now rising as steeply as they did in the last wave

Infections are currently doubling every nine to 10 days, and Javid said on Tuesday that the number of new daily cases could quadruple during the summer to reach 100,000. Although vaccines have cut the mortality rate from about 0.8 per cent before vaccination started to 0.1 per cent today, 100,000 cases would translate into about 3,000 hospital admissions and 100 deaths a day.

Covid on that scale would put the NHS under considerable strain, hospital leaders say. The extra beds and resources required to care for these patients would be diverted from clearing the non-Covid backlog.

The government is on track to achieve its target of offering every adult a first jab and to have given a second dose to two-thirds of adults by July 19, said Matt Linley, senior analyst with the life sciences analytics company Airfinity. It is also set to double-jab every adult by mid-September.

An even faster rollout would not be possible because of supply constraints over the two mRNA vaccines, from BioNTech/Pfizer and Moderna.

Chart showing that the government appears on course to meet its target of fully vaccinating two-thirds of adults by July 19th

In the UK’s favour is one of the world’s highest vaccine acceptance rates, said Airfinity chief executive Rasmus Bech Hansen. “The high uptake among the younger population has surprised us. Our models were more conservative.”

“It seems clear that the government policies are based on a new form of herd immunity strategy,” Kevin Courtney, joint general secretary of the National Education Union, said on Tuesday. “They are hoping that the increase in vaccination rates and the increase in infection rates across the summer will eventually get cases to fall simply because there is no one left to infect.”

The prime minister’s spokesman denied pursuing herd immunity, saying: “It’s not a policy goal of the government. We are moving through the age groups to provide first and second doses.”

Chart showing that Covid cases in the UK have climbed far above other countries

Herd immunity has become a dirty phrase for some people because it has connotations of letting infection rip through a population without caring sufficiently about the suffering caused.

But the policy is essentially to build up immunity in the population, aiming to provide as much as possible through vaccination rather than uncontrolled infection, so that eventually cases begin to fall as the virus encounters too few unprotected people to propagate further.

Additional reporting by George Parker