Never before in the 18-month history of Covid-19 have scientists been less confident about predicting the course of the pandemic. Projections of the future impact vary enormously, depending on the infectiousness of different viral variants, the effectiveness of vaccines at containing them, changing human behaviour and several other factors.

Many more people will inevitably die from Covid, though the number of deaths in the UK in the next 12 months could be a few thousand, tens of thousands or even hundreds of thousands. But scientists agree that the decision to postpone England’s final unlocking step by four weeks from Monday will substantially reduce mortality.

Despite this uncertain outlook, ministers have been arguing recently that Britain and the world will have to learn to live with what Cabinet Office minister Michael Gove called the “tragic consequences” of Covid, if society is to avoid future lockdowns. But there has been little discussion of the mortality level that might be broadly acceptable to the public.

Britain’s main disease modelling groups, whose advice informs government policy, all emphasise what Anne Cori of Imperial College London calls the “great uncertainty” surrounding their projections.

At a briefing on June 15 Cori presented estimates of total Covid deaths from now to June 2022, which varied tenfold from around 20,000 to 200,000. Key unknowns include the effectiveness of vaccines against Delta and other new variants, the durability of immunity conferred by coronavirus infection and vaccination, how transmissible the virus will become and the extent to which people maintain precautions such as mask wearing and social distancing after legal restrictions end.

“I think we’re at that tantalisingly difficult place where it’s too early to call victory,” said Sir Mark Walport, Sir Patrick Vallance’s predecessor as the government’s chief scientific adviser. “Everything is less categorical than earlier waves.”

Sir David Spiegelhalter, chair of the Winton Centre for Risk and Evidence Communication at Cambridge university, rejects the idea of “a single acceptable/unacceptable threshold” for deaths as the pandemic recedes. “There’s a grey area of risk we have to negotiate,” he said.

The Health and Safety Executive’s framework offers a precedent for how the government might define an acceptable level of Covid deaths in the long-term, according to Spiegelhalter.

The HSE defines an unacceptable level of risk for members of the public as an activity which causes one in 10,000 people to die per year, while an acceptable risk amounts to one death per million (quantified as one micromort). In between lies a wide “tolerable region” which Spiegelhalter believes the post-pandemic world will inhabit.

An increasing number of Conservative MPs believe that the pandemic has reached the acceptable level and lockdown restrictions should be eased.

Mark Harper, the former chief whip who leads the Covid Recovery Group of lockdown sceptical MPs, said “we must have an open society and proper ability for individuals to balance risks, we cannot hold society captive.”

“Of course, nothing is entirely safe but there’s a zone where we try to do our best to bring the risks down to an acceptable level,” Spiegelhalter added. “That’s a very powerful and successful way of managing risk and is essentially an official precedent for how we can manage Covid.” For the whole UK population the HSE’s tolerable zone would span from around 60 to 6,000 deaths per year.

But Stephen Reicher, professor of social psychology at St Andrews university, points out that public acceptance of risk depends critically on circumstances. “A few deaths caused by political mistakes or incompetence are far more outrageous than a large number that couldn’t possibly be avoided,” he said.

Although both Covid and flu are respiratory viral diseases, there are many clinical differences between them. Covid is more infectious than seasonal flu, more deadly and causes a wider range of symptoms.

A spike in Covid deaths will have to be avoided if it is to be comparable with flu, which kills around 11,000 people every winter in the UK. During the 2017/18 winter flu season, 22,000 people died of the virus, pushing UK death rates into ‘excess mortality’, but even that figure is only around a quarter of total Covid deaths over the past year.

Flu almost disappeared during the 2020/21 season, a fortuitous effect of social distancing during the pandemic.

Walport said flu offered “an obvious prototype” for an acceptable level of death, though such a dilemma “is not amenable to any sort of scientific answer”.

“This is really democracy in action,” he said, referring to the limits of advice scientists can give to government. “There comes a point at which science has to be overlaid with societal judgment as to what is and isn’t acceptable.”

Michael Parker, a bioethics professor at Oxford university, thinks so. “Talking about what number of deaths is acceptable is not something a politician can discuss but it is something that needs to be thought about,” he said. “It’s become obvious over the past year that these moral questions are at the heart of policymaking — this is not just about pandemics but it’s about the way we live.”

Mehrunisha Suleman, a bioethicist at the Health Foundation, said the strong public support for lockdown measures suggests people “will take some convincing” to accept more deaths in the long-term.

“In the balance of how much death we accept versus the freedoms we are willing to give up, people have consistently opted for reducing deaths,” she said. “I’m not sure that will change nor should it, as I worry our rush back to normality may trample over those harmed most by the pandemic.”

Across Whitehall, ministers are uneasy discussing the issue of what is an acceptable level of deaths. One senior official said: “It’s held very tightly, there are few official minutes kept because it’s one of the most difficult conversations we’ve had yet. There is a number, but obviously no one wants to discuss it.”