Scientists have urged the Government to set an “acceptable level” of Covid-19 infections.

Experts said they have been “crying out” for someone in a position of political power to decide what level of Covid-19 infections are acceptable so the outbreak can be “managed with that in mind”.

Meanwhile, Professor Dame Angela McLean, chief scientific adviser at the Ministry of Defence, said that unlocking society too fast “risks disaster”.

Official statistics predict that 695,400 people in England are estimated to have Covid-19.

MPs on the House of Commons Science and Technology Committee also heard that if a tier system is used after the current lockdown is lifted, it must be adapted.

They were also told that the data is “moving in the right direction” for the easing of restrictions to be considered.

Next week, the Prime Minister will set out his vision for easing the country out of lockdown and reopening various sectors of society.

Addressing the Committee, Dame Angela said: “I think we can say very, very clearly: ‘Don’t unlock too fast’, because if you unlock a lot, while a lot of the most vulnerable are still unvaccinated, genuinely we risk disaster, quite frankly.”

She also called for ministers to set an “acceptable” level of infections going forward.

Dame Angela said: “I think it’s reasonable to say: ‘Let’s not have Covid winters that are any worse than bad flu winters.’ But, actually, bad flu winters could be quite bad.

“It’s one of the things we’ve cried out for again and again – could somebody in a position of political power tell us what is an acceptable number of infections?

“Maybe this past year, maybe in 2020 where the number of infections and deaths was so high, perhaps nobody would say that.”

Dame Angela, who is a member of the Government’s scientific advisory group Sage, and also co-chair of the SPI-M Sage sub group, added: “We do need to decide what level is acceptable and then we can manage our lives with that in mind.”

Professor Mark Woolhouse, professor of infectious disease epidemiology at the University of Edinburgh, added: “The answer is not zero.

“If you take the view that no Covid death is acceptable or something of that order, you are writing a blank cheque to do any amount of harm by the measures you have implemented to try and control it.”

He added there does not appear to be a route to a “New Zealand-like position”.

But Prof Woolhouse added that the data is pointing to “earlier unlocking”.

“I completely agree that we don’t want to be overly focused on dates, not at all. We want to be focused on data. But the point I’d make about that is the data are going really well,” he said.

“The vaccination rollout is, I think, exceeding most people’s expectations, it’s going very well.

“The transmission blocking potential is key. But so, of course, is its actual ability to protect against death and disease, and to keep people out of hospital, and those numbers are looking really good.

“My conclusion from that is: If you’re driven by the data and not by dates, right now, you should be looking at earlier unlocking.”

Dame Angela added: “Things are all moving in the right direction: infections are falling, the number of cases are falling, hospitalisations and deaths.

“But we still stand with a high number of infections. I share everybody’s optimism about how fantastic this vaccine is. But I would say we need to be optimistic and cautious, there’s still a lot of infected
people out there.”

When probed about the tier system, Dame Angela added: “It was the way we used the tier system – we waited until prevalence was high before putting it into a more restrictive tier.

“What we should have done was say: ‘Look, this part of the country, the number of infections is starting to grow’, and (we should have) put them into higher tiers while their infection (rates) were still low.”

She added: “It was the way that we used the tier system that we need to do different this time – to use the tiers to act when prevalence is low but growing.”

Prof Woolhouse added: “The earlier you act, the less drastic you have to act.”