Coronavirus: Cambridge experts help predict what will happen to Covid infection rates after lockdown ends
Returning to the previous tiered system of restrictions post-lockdown will cause Covid-19 infections to rise back to levels seen early in November, the government’s scientific advisers have warned.
In a set of new documents released by the Scientific Advisory Group for Emergencies (Sage) on Friday, the experts said that if the four-week lockdown measures are well adhered to, the number of hospital admissions and deaths can be expected to fall until at least the second week of December.
But they added a longer-term outlook will depend on both the nature of non-pharmaceutical interventions implemented after December 2, when the lockdown measures are expected to be lifted, and policies over the festive period.
The group’s document, dated November 4, said: “If England returns to the same application of the tiering system in place before November 5, then transmission will return to the same rate of increase as today.”
The Sage group features three University of Cambridge academics:
- Professor Julia Gog, from the Department of Applied Mathematics and Theoretical Physics, who models the spread of infectious diseases;
- Professor Dame Theresa Marteau, director of behaviour and health research unit, who evaluates interventions to change behaviour; and
- Professor Sir David Spiegelhalter, a statistician who chairs the Winton Centre for Risk and Evidence Communication.
A sub-group of Sage - the Scientific Pandemic Influenza Group on Modelling (SPI-M) - set out three scenarios in a document dated October 28.
It said: “None of these scenarios are palatable.
“Nevertheless, until such a time that more effective treatments are available or a significant proportion of the population have been immunised, no other scenarios are possible.”
In one scenario where prevalence is “low and controlled”, and where NHS Test and Trace “can play a big role in containing outbreaks”, there is scope for a loosening of social distancing rules over Christmas for a “limited time”, the experts said.
For this to happen, “rapid and decisive interventions” must push R well below one and “maintain that for some time”.
However, the scientists noted that the only time this has happened so far was during the March/April lockdown.
In a different “high and controlled” cases scenario, which experts believe the UK is in at the moment but with potential for cases to drop, there is “little to no scope for loosening of social distancing rules over Christmas”.
This scenario is when infection rates are high for several months and too high for NHS Test and Trace to be effective.
A third scenario predicts a much worse outcome but occurs only when government interventions “are not sufficient to stop epidemic growth”, SPI-M said.
In a note of optimism, in a document dated November 4, SPI-M said the current lockdown restrictions are likely to push R under one if they are “well-adhered to”.
But SPI-M said stricter controls will then be needed than the three-tier system that was in place before lockdown.
On October 12, Prime Minister Boris Johnson announced England would be placed into “medium”, “high” and “very high” alert levels – or tier 1, 2 and 3 – involving varying restrictions aimed at tackling the virus.
But as the number of coronavirus infections continued to rise, a four-week lockdown was introduced on November 5.
Meanwhile, the latest report from the MRC Biostatistics Unit at the University of Cambridge estimates the number of new infections occurring daily to be 64,200, with the Midlands (20,100 daily) and the North West (12,600 daily) worst affected, although they note many of these cases are likely to be asymptomatic.
They said on Wednesday (November 11) that the R number - indicating how many people an infected person passes on the virus to - is above one in all regions “with almost 100 per cent probability”, although it is plateauing, they noted, with “downward trends in the North East and Yorkshire and the North West”. This may be the result, they suggested, of social distancing measures, but they warned the impact was “not strong enough” to reduce the R number values below one, although a different study by King’s College suggested it might be in the North-West.
The MRC Biostatistics Unit warned the number of infections is growing five per cent daily, explaining: “This translates into a doubling in the total number of new infections approximately every 14 days, although there is regional heterogeneity in the doubling times.
“London, followed by the North West, continues to have the highest attack rate, that is the proportion of the population who have ever been infected, at 21 per cent and 20 per cent respectively. The South West continues to have the lowest attack rate (five per cent).
Prof Daniela De Angelis, programme leader and deputy director of the unit, and a member of one of the SPI-M, said: “The R values have slightly decreased in all regions over the half-term period and then reverted to pre-half term trends.
“In general, they seem to be slowly decreasing, but remain above one in all regions. The number of infections is estimated to be around 65,000 per day, indicating a downwards revision from estimates last week. This results from a temporary decrease in activity over the half-term period.
“It is too early to observe the effect of the lockdown, so the number of infections is still predicted to increase and lead to a raise in mortality in the near future. We continue to monitor the situation closely.”
The data comes as figures show the number of cases rose in the seven days to November 8 in Cambridge (215, up 33), South Cambridgeshire (171, up 70), East Cambridgeshire (70, up 36), Huntingdonshire (188, up 68) and Peterborough (382, up 83). Only Fenland (77, down 11) recorded a fall.
It was during this seven-day period that the second lockdown began, meaning the infections will have pre-dated the period, underscoring the reason for the restrictions.
Meanwhile, another Sage document suggests the national outbreak is still at a “high and controlled” phase.
If this remains the case, or if the outbreak returns to current levels after the lockdown, Sage has advised there is “little to no scope for loosening of social distancing rules over Christmas”.
But if prevalence is “low and controlled” and R is “well below one” for some time, the experts say there may be a “greater potential for loosening of social distancing rules for a limited period of time during the festive period”.
The government scientists said if R is reduced to 1.1 or lower for some time, there may be a “limited accumulation of population immunity”.
This, they say, will start reducing the average population susceptibility to the virus and slow down transmission.
Sage said: “When R is 1.1, only nine per cent of the remaining susceptible (ie not previously infected) population need to be infected for R to fall to one, solely as a result of the natural dynamics of the epidemic. At this point, in some sense, population immunity has caused the epidemic to plateau.”
However, they added that population immunity is “very different from a classic ‘herd immunity’ scenario, where an epidemic has run through a population with limited impact of control measures”.
The experts said in a population immunity scenario, there “will be very limited room to relax interventions, since the absolute level of population immunity reached will likely still be low”.
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