Imperial College London

England’s COVID-19 roadmap successful in balancing transmission and immunity

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England’s COVID-19 roadmap was successful in balancing increased transmission with population immunity, according to a major study.

The roadmap out of lockdown managed transmission rates by balancing increasing contact rates from the gradual re-opening of society with growing population immunity through vaccination, according to new analysis from Imperial’s COVID-19 Response Team.

Dramatic risk reduction

Delaying the lifting of all restrictions by a month to July 19, in response to Delta’s emergence, is also likely to have dramatically reduced the risk of a large autumn/winter peak in hospital admissions and reduced deaths in the 12 months following June 2021 by around 20%.

While hospitalisations and deaths are still a fraction of what they were in January, Delta means that infections are currently higher than we would have hoped for. Prof Neil Ferguson

The analysis, published in The Lancet, estimates that lifting all non-pharmaceutical interventions (NPIs) on 21 June 2021, as originally planned, might have led to over 3,400 peak daily hospital admissions due to the impact of the Delta variant. Delaying to 19 July reduced the potential peak in admissions by approximately three-fold.

Roadmap out of lockdown 

The government’s roadmap out of lockdown for England saw four incremental steps of easing of restrictions, beginning on 8 March and finishing with all restrictions lifted from 19 July.

The researchers say that following step 1 of the roadmap on 8 March, when schools and educational institutions re-opened, the numbers of cases, hospitalisations and deaths continued to fall, as had been the trend during the full lockdown in the spring.  

Following step 2 on 12 April, the R number for Alpha continued to be below one due to increasing population immunity through vaccination. The researchers estimate that the R number for Alpha remained below one through to July 2021.

Delta variant

If Delta had not emerged, the planned roadmap out of lockdown would have been sufficient to keep cases, severe infections, and deaths low and manageable. Dr Raphael Sonabend

The Delta variant was detected in England in early April and drove a rapid increase in cases and hospitalisations from mid-May that was not entirely counterbalanced by the vaccination programme due to the increased transmissibility of the variant and vaccines being less effective against it.

The increase in contact rates following step 3 continued to be partially offset by the increasing immunity in the population through the vaccine rollout. However, while the effective R number for Alpha remained below one, for Delta it remained above one.

The projections show that had the Delta variant not emerged, lifting all restrictions as planned on 21 June would not have led to a substantial third wave.

Euro football tournament

A sharp increase in transmission was seen in early July followed by a rapid drop following July 11. This suggests that the increase in transmission was probably primarily caused by a change in social contact rates, likely associated with the Euro football tournament, and not as a result of step 3 of the roadmap. This surge in cases, as well as evidence that Delta was more transmissible and potentially more severe than Alpha, prompted the delay of step 4 by a month until July 19. This allowed the additional distribution of 2.8 million first and 3.8 million second vaccine doses.  

Recent trends in cases, hospitalisations and deaths up to mid-August suggest that contact rates have increased gradually since Step 4 on July 19.

Autumn and winter projections

The team project an autumn wave of transmission, but with large uncertainty around the resulting peak hospitalisations and total deaths. This uncertainty is driven by imperfect knowledge of vaccine effectiveness against Delta, the overall level of population immunity, accounting for waning of natural immunity and imperfect cross-protection between variants, and the level of social mixing.

The team says that the transmission intensity in the coming months will depend on how high and how quickly population contact rates will increase, the roll-out of booster doses and vaccination of teenagers, and adherence to case isolation.

Dr Raphael Sonabend said: “At each stage of the COVID-19 roadmap, we modelled a wide range of scenarios to analyse how lifting restrictions may change the course of the epidemic."

“We found that lifting restrictions in July, a month later than originally planned, successfully reduced the risk of a large autumn wave of infection, hospitalisations and deaths. If Delta had not emerged, the planned roadmap out of lockdown would have been sufficient to keep cases, severe infections, and deaths low and manageable. The latest data demonstrates the importance of a gradual increase in social mixing back to pre-pandemic levels. We remain cautiously optimistic and will continue to use our models to monitor the epidemic.”

Professor Neil Ferguson said: "Our analysis shows that timing the gradual relaxation of COVID restrictions to track the roll-out of vaccination was largely successful at limiting infection levels, albeit some additional challenges were posed by the rise of the Delta variant in May this year. While hospitalisations and deaths are still a fraction of what they were in January, Delta means that infections are currently higher than we would have hoped for. Rapid roll-out of booster doses and second doses in teenagers is likely to be key to maintaining control of transmission in the next few months."

Dr Marc Baguelin said: "In our study, we’ve shown that, despite the lifting of restrictions, until the emergence of Delta in June, the R number had remained under or close to 1 thanks to the vaccination programme. With the sudden surge of cases of the Delta variant, mathematical models gave a strong signal that a month delay was needed to catch up with vaccinations and get the roadmap back on track. While infections are still high, we are now in a much better place than a year ago. One of the lessons of our work is that mathematical models can be extremely useful in informing policy makers when an open and transparent collaboration is put in place."

Dr Anne Cori said: “The aim of the roadmap policy was to relax restrictions in a gradual manner that would mirror and be offset by the gradual increase of population protection through vaccination, so that new infections would remain at reasonably low levels. Our study shows that this aim would have been achieved if Delta had not emerged. But Delta completely changed this balance between population protection and social mixing, because vaccines were slightly less effective against it, and most importantly, Delta was far more transmissible than Alpha."

"Our study illustrates the role mathematical models can play to help inform the design, monitoring and evaluation of policies such as the roadmap. It also emphasises the potential threat posed by new variants, and the need for policies to be reviewed on a regular basis in light of new scientific evidence. We estimate that delaying step 4 of the roadmap following the emergence of Delta reduced the peak in hospitalisations by a factor of 3.”    

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Stephen Johns

Stephen Johns
Communications Division

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Email: s.johns@imperial.ac.uk

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Dr Sabine L. van Elsland

Dr Sabine L. van Elsland
School of Public Health

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Tel: +44 (0)20 7594 3896
Email: s.van-elsland@imperial.ac.uk

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