Modelling Step 4 of the UK Government’s Spring 2021 “Roadmap” out of lockdown in England

All scenarios project a new wave of infection if Step 4 had been enacted on 21st June
Illustration of COVID-19. Credit: CDC/Alissa Eckert

Enacting Step 4 of the UK Government’s ‘roadmap’ out of lockdown on 21 June could have led to a third wave of COVID-19 infection that was comparable to the April or October 2020 waves in terms of hospitalisations and deaths, according to new projections by the London School of Hygiene & Tropical Medicine (LSHTM).

The research was conducted by a team from LSHTM’s Centre for the Mathematical Modelling of Infectious Diseases (CMMID) and is yet to be peer reviewed. The report, along with work from Imperial College London and the University of Warwick, is informing the UK government's ‘roadmap’ out of lockdown for England.

The team used a model to project the effect of both implementing Step 4 on the 21st of June 2021, as originally planned, and delaying it, using various combinations of assumptions related to the levels of immune escape and increased transmissibility of the Delta B.1.617.2 variant compared to the Alpha B.1.1.7 variant.

In a medium immune escape and 50% increase transmissibility scenario implementing Step 4 on 21st June could have led to a third wave peaking in August or early September 2021, resulting in a projected 49,700 deaths (estimate range 35,600-67,200) by the end of October 2021.

A two-week delay would reduce deaths to 48,500 (34,400-66,900). Delaying Step 4 for five weeks, so that it coincides with the school vacation period, was expected to have a larger effect in the short term, reducing the number of deaths to 43,500 (31,200-62,900) and peak deaths from 700 (500-1,200) per day to 500 (300-800).

Taking further measures to reduce the summer wave, such as delaying Step 4 until all adults have had both vaccine doses, instigating a circuit-breaker lockdown or moving back to Step 2 measures, is likely to largely negate the impact of the summer wave. This is unless pessimistic assumptions regarding immune escape and transmissibility are adopted.

The model projects that more than half of the hospitalisations and deaths occurring in the summer 2021 wave will be in unvaccinated individuals, with admissions being split relatively evenly between the 45-64, 65-74 and 75+ year age groups. Deaths are likely to be concentrated in the 75+ age group

The researchers say there is considerable uncertainty over their projections with data on the properties of the Delta variant strengthening all the time, and the size of the peak depending strongly upon social mixing which is very difficult to estimate.

Dr Rosanna Barnard from CMMID led the analysis. She said: “Our projections suggest there will be a third wave of COVID-19 infection in England this summer. While the successful vaccine roll-out continues apace, the increased transmissibility of the Delta variant, along with emerging evidence that it evades vaccines to some degree, means that any increase in social mixing is likely to result in more infections, hospitalisations and sadly deaths.”

“From an epidemiological point of view, the delay to enacting step 4 of the roadmap is sensible.  Our projections show that enacting between a 2- and a 9-week delay to step 4 can substantially reduce peak hospitalisations and deaths, but it is also important to acknowledge that more restrictions have a detrimental effect on people’s lives and livelihoods, especially those working in the hospitality sector.”

***This study is not yet peer-reviewed***


LSHTM: Interim roadmap assessment - prior to Step 4, 9 June

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