Government modellers have admitted they do not know whether releasing restrictions on June 21 would lead to unsustainable pressure on the NHS, as experts said the situation "might be a lot better" than previous waves.
As Boris Johnson announced a delay to the full lifting of restrictions, scientists admitted there was still "huge uncertainty" about whether Britain would experience a devastating third wave.
They warned that models had struggled to "reconcile" the rapid growth in delta variant cases with "largely flat admissions" in hospitals, and said delaying reopening would help them to "resolve these data interpretation issues".
Government scientific experts also said Britain may now be just "one smallish wave" away from the end of the pandemic.
Impact of Delta variant infections in England
Models released from Warwick University, Imperial College and the London School of Hygiene and Tropical Medicine (LSHTM) on Monday show vast differences in the path the pandemic might take over the coming months.
Under the most pessimistic scenarios, Imperial modellers warned Britain could experience a further 203,824 deaths by next June, around 50,000 more deaths than the first and second waves combined.
All models show a significant wave of infections in the summer, with Warwick suggesting a maximum of 5,990 admissions per day by July 22, which would be higher than the 4,000 peak of the January wave.
Warwick also warned deaths could peak at 1,780 per day (again, higher than the first and second waves) leading to a third wave total of 72,400 by next June.
How different scenarios impact hospitalisations
All the models suggest that a delay of several weeks would bring the deaths down by around 5,000 but will not stop the majority of third wave deaths.
Under a central scenario, the LSHTM estimates that there would be 38,400 deaths by Oct 31 without any pause, but that would fall to 33,200 (a drop of 5,200) with a five week delay.
Likewise, Imperial estimates that a delay would see its central third wave death estimates fall from 47,941 to 42,060 (a decrease of 5,881), while Warwick estimates that hundreds of daily admissions could be cut with a delay of several weeks.
Overall, models suggest that a four-week delay reduces the peak in hospital admissions by around a third to a half.
However, in a summary document from the Government’s Scientific Pandemic Influenza Group on Modelling (SPI-M), experts admitted the scale of a resurgence is "highly uncertain" and said it may be "considerably smaller or larger than previous waves".
The conclude: “SPI-M cannot determine with confidence whether taking Step 4 of the Roadmap on June 21 would result in a peak that might put unsustainable pressure on the NHS.
“The epidemic is at a particularly uncertain point in time and models have struggled to reconcile the rapid growth in delta cases with what appear to be currently flat hospitalisations.
Affect of lifting step 4 of road map
“While there is a significant resurgence in admissions in all scenarios, the scale of that resurgence is highly uncertain and ranges from considerably smaller than January 2021 to considerably higher.”
SPI-M has advised that delaying Step 4 would "enable more time to resolve these data interpretation issues" and accrue more information on the delta variant’s transmission and hospitalisation rates".
Latest data shows there has been a 45 per cent increase in infections in the last week, a 15 per cent rise in admissions, and 11.9 per cent in deaths, although death numbers although all are still low.
A scientific source close to the Government said: “We are in a so much better place now. The numbers are all much smaller. Things might be a lot better than either of the waves of the last 12 months or they could be worse.
Effectiveness of vaccines against hospitalisation (PHE)
“The benefit of the vaccine is that the proportion ending up in hospital is much smaller. Really this is telling us we need to get more vaccines into as many individuals as possible.
“The over-60 case rate is not climbing and even if they are infected, there is a reduction in hospitalisation. We may be one smallish wave away from the beginning of the end.
The fully vaccinated are still at risk from Covid-19
“If we’re lucky nothing will happen. There is massive uncertainty here, but things could have been so much worse without the vaccination campaign. It doesn’t bear thinking about.”
Speaking about the modelling, Sir Patrick Vallance, the Government’s chief scientific adviser, said: “The four-week delay should reduce the peak by 30 and 50 per cent, that’s the sort of gain we get, and it’s not very obvious you get much more benefit from going on longer.
“If we didn’t have the vaccination we’ve got we would be looking is more lockdown needed. That’s not where we are. We’re in a race against the virus and the vaccines need to get ahead of it. If you’re in a race you don’t assist them in putting the afterburners on so they can outpace you.”
Prof Rowland Kao, the Sir Timothy O’Shea professor of veterinary epidemiology and data science, University of Edinburgh, said: “The considerable uncertainty associated with the outcomes is a real indicator of the uncertainty in the data – there is much we simply don’t know.
“Any decisions announced reflect that uncertainty, and thus it is entirely possible that the outcome over the next few weeks will be substantially fewer cases and deaths than the extremes of the modelling scenarios.”