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Non-pharmaceutical interventions, vaccination, and the SARS-CoV-2 delta variant in England: a mathematical modelling study

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Title: Non-pharmaceutical interventions, vaccination, and the SARS-CoV-2 delta variant in England: a mathematical modelling study
Authors: Sonabend, R
Whittles, LK
Imai, N
Perez Guzman, PN
Knock, E
Rawson, T
Gaythorpe, KA
Djaafara, A
Hinsley, W
Fitzjohn, R
Lees, JA
Thekke Kanapram, D
Volz, E
Ghani, A
Ferguson, NM
Baguelin, M
Cori, A
Item Type: Journal Article
Abstract: Background: England's COVID-19 roadmap out of lockdown policy set out the timeline and conditions for the stepwise lifting of non-pharmaceutical interventions (NPIs) as vaccination roll-out continued, with step one starting on March 8, 2021. In this study, we assess the roadmap, the impact of the delta (B.1.617.2) variant of SARS-CoV-2, and potential future epidemic trajectories. Methods: This mathematical modelling study was done to assess the UK Government's four-step process to easing lockdown restrictions in England, UK. We extended a previously described model of SARS-CoV-2 transmission to incorporate vaccination and multi-strain dynamics to explicitly capture the emergence of the delta variant. We calibrated the model to English surveillance data, including hospital admissions, hospital occupancy, seroprevalence data, and population-level PCR testing data using a Bayesian evidence synthesis framework, then modelled the potential trajectory of the epidemic for a range of different schedules for relaxing NPIs. We estimated the resulting number of daily infections and hospital admissions, and daily and cumulative deaths. Three scenarios spanning a range of optimistic to pessimistic vaccine effectiveness, waning natural immunity, and cross-protection from previous infections were investigated. We also considered three levels of mixing after the lifting of restrictions. Findings: The roadmap policy was successful in offsetting the increased transmission resulting from lifting NPIs starting on March 8, 2021, with increasing population immunity through vaccination. However, because of the emergence of the delta variant, with an estimated transmission advantage of 76% (95% credible interval [95% CrI] 69–83) over alpha, fully lifting NPIs on June 21, 2021, as originally planned might have led to 3900 (95% CrI 1500–5700) peak daily hospital admissions under our central parameter scenario. Delaying until July 19, 2021, reduced peak hospital admissions by three fold to 1400 (95% CrI 700–1700) per day. There was substantial uncertainty in the epidemic trajectory, with particular sensitivity to the transmissibility of delta, level of mixing, and estimates of vaccine effectiveness. Interpretation: Our findings show that the risk of a large wave of COVID-19 hospital admissions resulting from lifting NPIs can be substantially mitigated if the timing of NPI relaxation is carefully balanced against vaccination coverage. However, with the delta variant, it might not be possible to fully lift NPIs without a third wave of hospital admissions and deaths, even if vaccination coverage is high. Variants of concern, their transmissibility, vaccine uptake, and vaccine effectiveness must be carefully monitored as countries relax pandemic control measures.
Issue Date: 13-Nov-2021
Date of Acceptance: 6-Oct-2021
URI: http://hdl.handle.net/10044/1/92384
DOI: 10.1016/S0140-6736(21)02276-5
ISSN: 0140-6736
Publisher: Elsevier
Start Page: 1825
End Page: 1835
Journal / Book Title: The Lancet
Volume: 398
Issue: 10313
Copyright Statement: © 2021 The Author(s). Published by Elsevier Ltd. This is an Open Access article under the CC BY 4.0 license (https://creativecommons.org/licenses/by/4.0/)
Sponsor/Funder: Medical Research Council (MRC)
National Institute for Health Research
International Society for Infectious Diseases
Medical Research Council
Funder's Grant Number: MR/R015600/1
Keywords: Science & Technology
Life Sciences & Biomedicine
Medicine, General & Internal
General & Internal Medicine
General & Internal Medicine
11 Medical and Health Sciences
Publication Status: Published
Online Publication Date: 2021-10-28
Appears in Collections:Department of Infectious Diseases
Faculty of Medicine
Imperial College London COVID-19
School of Public Health

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