Britain may not be able to abandon social distancing rules this year unless a vaccine proves to be 85 per cent effective at stopping transmission of coronavirus as well as severe illness, ministers have been warned.
Modelling commissioned by SPI-M, a subgroup of Sage, and passed to Downing Street suggests the UK will suffer a third huge spike in deaths unless inoculation cuts transmission significantly.
Currently, most experts think efficacy against transmission will be around 60 per cent but there is huge uncertainty.
The paper, produced by modellers at the University of Warwick, warns: "Only vaccines that offer high infection-blocking efficacy with high uptake in the general population allow relaxation of non-pharmaceutical interventions without a huge surge in deaths."
The paper, published last week, makes for grim reading and goes some way to explaining why Boris Johnson has been so reluctant in recent weeks to raise expectations of an end to lockdown.
It is understood that SPI-M's other modelling groups – including Imperial College London and the London School of Hygiene and Tropical Medicine – have reached similar conclusions.
"The exact numbers differ, but there is a high degree of consensus", a source told The Telegraph.
The paper finds that even in a best case scenario, in which vaccines stop 85 per cent of transmission in those vaccinated, lockdown would have to be kept in place until the end of May to prevent another significant spike in deaths.
If Mr Johnson lifted the measures in mid-February – when ministers forecast that the top four priority groups will be vaccinated – the modelling suggests a third surge in infections and deaths, of a scale similar to last spring, would follow in April.
If vaccines are 85 per cent transmission blocking, deaths would peak at just under 1,000 a day. At 60 per cent efficacy, daily deaths would approach 1,500.
"While the novel vaccines against SARS-CoV-2 offer a potential exit strategy for this outbreak, this is highly contingent on the infection-blocking (or transmission-blocking) action of the vaccine and the population uptake, both of which need to be carefully monitored as vaccine programmes are rolled out in the UK and other countries," the authors say.
Dr Sam Moore, a post-doctoral research associate at Warwick and one of the paper's authors, said a number of "non-obvious" factors were driving the numbers.
He said that because the natural 'R' rate of the virus was so high – above three – even vaccines that provided "very high" infection blocking would struggle to get the 'R' number below one in the absence of continued social distancing measures.
"There is definitely a sort of threshold," he said. "If we get very high infection blocking then this could almost be what delivers us from it, but it would have to be very high. Even vaccines with 85 per cent infection blocking are insufficient to drive 'R' below one on their own."
Unless 'R' falls below one, the incidence of the virus continues to grow exponentially.
The number of people who remain vulnerable to disease will also remain high for much longer than might be immediately obvious, said Dr Moore. The efficiency of the vaccines in preventing severe disease, the time between doses and overall vaccine uptake all add up.
"The vaccines are not going be 100 per cent effective at stopping serious disease. So if you manage to get, say, 85 per cent of people to take it and it turns out to be 90 per cent effective, that's still 25 per cent of people who could die from it, which is a lot of people," he said.
Nadhim Zahawi, the vaccines minister, said he had not read the Warwick paper but was aware of the issue. Ministers had prioritised work through Public Health England to understand the impact vaccines would have on transmission, he added.
"We're probably another four weeks away from getting some of that data back but it should be able to then allow us to make plans… we'll be able to see the impact of vaccines on transmission and also on hospitalisation and serious infection," Mr Zahawi said.
The Warwick paper models different scenarios for unwinding social distancing. Which one is ultimately chosen by ministers will hinge on the efficacy of the vaccines in preventing transmission, something likely to differ between products.
If the vaccines provide an average 60 per cent block on transmission, which experts hope, then ending lockdown at the end of May and reverting to the social distancing measures that were in place in early September produces the fewest deaths in the Warwick model.
However, those restrictions – which included a prohibition on large events – would have to be maintained until the end of the year.
A Government spokesman said: "At this early stage in the vaccination programme, scientists do not have sufficient data to advise how the vaccine may affect onward transmission.
"As large numbers of people from at risk groups are vaccinated, we will be able to gather the evidence to asses the impact on infection rates, hospitalisation and reduced deaths. It's important to continue following the national restrictions, instructions from NHS Test and Trace and to self-isolate if you are instructed to do so, even if you have had the vaccine."
