At a long and difficult cabinet meeting on Monday, hawkish ministers demanded “incontrovertible evidence” that Omicron risks overwhelming the NHS to justify the cost of taking action, while on Tuesday Boris Johnson confirmed his view that there is insufficient evidence to justify new measures – yet.

While some clarity on the hospitalisation rate associated with the new variant should be available within a day or two, scientists have warned that getting the evidence ministers want may be “a tall order”. So why is it so hard to work out, and what does that mean for decision-making?

Omicron cases are doubling quicker than every 48 hours – which means the number of individuals who will develop severe disease is also rising exponentially. However, it takes roughly 10 days from testing positive for Covid for this increase to be observed in the hospital data.

Ten days ago, the total number of confirmed Omicron cases in the UK was 1,898. As of 19 December, the cumulative total was 45,145 – although the true number will be many times higherbecause a lot of infected people are not getting tested and there is a delay in confirming suspected Omicron cases. Hospitals in London are reporting an uptick in Covid admissions but 10 days from now the number is likely to have mushroomed, regardless of Omicron’s severity, because of the sheer number of infections.

Many scientists remain sceptical of claims Omicron triggers less severe disease. More certain is that many of those who have been vaccinated will experience less severe disease, meaning fewer of these individuals will be admitted to hospital. Yet pinpointing precisely how much impact vaccinations – and especially boosters – will have is fraught with difficulty.

Modelling studies help, but these tend to predict a range of scenarios. “You go from something that’s not as bad as last winter, to much worse. From a policymaking point of view, that’s not that helpful, because they don’t assign any probability to any of those scenarios,” said Dr Raghib Ali, a senior clinical research associate at the University of Cambridge MRC Epidemiology Unit.

“I think incontrovertible evidence is a tall ask,” said Prof Sheila Bird, formerly of the Medical Research Council’s biostatistics unit at the University of Cambridge. “What I imagine [minsters] are trying to do is work out [what] is the effect of the booster immunisations on hospitalisation rates for Omicron versus Delta. But that depends upon how many people were boosted about four weeks ago, then had two weeks to develop full protection from that booster, in that time got Omicron versus got Delta, and then were hospitalised.”

Part of the problem is that things are constantly changing: the rate at which people have been receiving boosters has changed, while fear about Omicron’s spread has prompted some people to behave more cautiously or to seek PCR tests in greater numbers.

Others have said that risk to the NHS and other sectors of society isn’t so much about disease severity as the extraordinarily high number of infections. “It is transmission that drives impact more than severity,” Jeremy Farrer, the director of the Wellcome Trust and a former member of Sage, told the Today programme. He said the country was in “the most difficult, most uncertain” period of the pandemic so far.

Even with high uptake of booster vaccines, there will be some who remain more vulnerable to severe disease, and not everyone has been boosted or had time for those boosters to take effect. Then there are roughly 6 million adults who haven’t been vaccinated at all. “That’s a lot of people, when the level of transmission is what it is today,” Farrer said.

Uncertainties remain, but Ali believes a clearer picture should emerge imminently. “The key metric, which takes into account both the intrinsic severity of Omicron and the degree of human escape, is the case hospitalisation rate – so, for a given number of cases, what proportion ended up in hospital?” he said. “With Delta, that’s currently about 2%. With Omicron, we don’t know yet, but I’d have thought that should be coming by this week, because there have been enough cases now.”

Yet even this estimate will be only a rough guess, due to the small number of cases and their demographics.

Dr David Pascall, a postdoctoral research associate, also at the MRC Biostatistics Unit, said: “Estimates can be made in real time, but the early estimates will be imprecise and biased. Stable precise estimates will be only possible to generate early next year. Probably in the latter half of January.

“At the moment, Omicron cases have been rising most in younger age groups who are likely to have less negative outcomes on infection. We have reasons to expect that this won’t last, not least because of the increased intergenerational mixing expected over the Christmas period. This will have unpredictable impacts, given the questions still remaining over the effectiveness of vaccine protection against Omicron.

“As such, we can have expectations about how severe infections in this wave will be, but incontrovertible evidence on both absolute severity and severity relative to Delta (over the same time period) can only be generated retrospectively after hospitalisations occur.”

However, concern isn’t limited to the NHS being overwhelmed. High rates of absence in key sectors such as education, law enforcement, or food processing and delivery could have unforeseen impacts on our lives in the coming weeks. Dr Simon Clarke, an associate professor in cellular microbiology at the University of Reading, said: “People understandably focus on hospitalisations and deaths, but we should not overlook the effects of a mass sickness on public services, infrastructure and economic activity.”

And then there’s long Covid: infections need to be minimised to prevent the risk of long-term disability and suffering, experts warn.

Waiting for incontrovertible evidence is a high-stakes gamble. The government placed a similar bet in March 2020 when it delayed introducing measures as other countries locked down. It lost, resulting in months of restrictions, hardship and unnecessary deaths. The UK is in a better situation now, thanks to the vaccines, but Omicron has added an extra dimension of uncertainty. It’s by no means certain ministers won’t lose again.