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.

Funding

National Institute for Health Research, UK Medical Research Council, Wellcome Trust, and UK Foreign, Commonwealth and Development Office.

中文翻译：

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英国的非药物干预、疫苗接种和 SARS-CoV-2 三角洲变异：一项数学模型研究

背景

英格兰解除封锁政策的 COVID-19 路线图规定了随着疫苗接种的继续推广逐步取消非药物干预 (NPI) 的时间表和条件，第一步从 2021 年 3 月 8 日开始。在这项研究中，我们评估路线图、SARS-CoV-2 的 delta (B.1.617.2) 变体的影响，以及未来可能的流行轨迹。

方法

进行这项数学建模研究是为了评估英国政府放松英国英格兰封锁限制的四步过程。我们扩展了先前描述的 SARS-CoV-2 传播模型，将疫苗接种和多菌株动力学结合起来，以明确捕捉 delta 变体的出现。我们使用贝叶斯证据综合框架将模型校准为英语监测数据，包括入院、住院率、血清阳性率数据和人口水平 PCR 检测数据，然后针对一系列不同的放宽 NPI 的时间表对流行病的潜在轨迹进行建模. 我们估计了由此产生的每日感染和住院人数，以及每日和累计死亡人数。从乐观到悲观的疫苗有效性的三种情景，研究了自然免疫力减弱和先前感染的交叉保护。我们还考虑了解禁后的三级混合。

发现

路线图政策成功地抵消了从 2021 年 3 月 8 日开始取消 NPI 导致的传播增加，并通过疫苗接种提高了人口免疫力。然而，由于 delta 变体的出现，与 alpha 相比，估计传输优势为 76%（95% 可信区间 [95% CrI] 69–83），因此在 2021 年 6 月 21 日完全取消 NPI，如原计划可能有在我们的中心参数情景下，导致 3900（95% CrI 1500–5700）每日高峰入院。推迟到 2021 年 7 月 19 日，住院高峰期减少了三倍，达到每天 1400 人（95% CrI 700–1700）。流行轨迹存在很大的不确定性，对 delta 的传播能力、混合程度和疫苗有效性的估计特别敏感。

解释

我们的研究结果表明，如果放宽 NPI 的时间与疫苗接种覆盖率仔细权衡，则可以大大降低因取消 NPI 而导致大量 COVID-19 住院的风险。然而，对于 delta 变体，即使疫苗接种覆盖率很高，也可能无法在没有第三波住院和死亡的情况下完全解除 NPI。随着各国放松大流行控制措施，必须仔细监测关注的变异、它们的传播性、疫苗接种和疫苗有效性。

资金

国家健康研究所、英国医学研究委员会、威康基金会和英国外交、联邦和发展办公室。