Impact of the Tier system on SARS-CoV-2 transmission in the UK between the first and second national lockdowns

Objective

Measure the effects of the Tier system on the COVID-19 pandemic in the UK between the first and second national lockdowns, before the emergence of the B.1.1.7 variant of concern.

Design

Modelling study combining estimates of the real-time reproduction number R _t (derived from UK case, death and serological survey data) with publicly available data on regional non-pharmaceutical interventions. We fit a Bayesian hierarchical model with latent factors using these quantities, to account for broader national trends in addition to subnational effects from Tiers.

Setting

The UK at Lower Tier Local Authority (LTLA) level.

Primary and secondary outcome measures

Reduction in real-time reproduction number R _t .

Results

Nationally, transmission increased between July and late September, regional differences notwithstanding. Immediately prior to the introduction of the tier system, R _t averaged 1.3 (0.9 – 1.6) across LTLAs, but declined to an average of 1.1 (0.86 – 1.42) two weeks later. Decline in transmission was not solely attributable to Tiers. Tier 1 had negligible effects. Tiers 2 and 3 respectively reduced transmission by 6% (5%-7%) and 23% (21%-25%). 93% of LTLAs would have begun to suppress their epidemics if every LTLA had gone into Tier 3 by the second national lockdown, whereas only 29% did so in reality.

Conclusions

The relatively small effect sizes found in this analysis demonstrate that interventions at least as stringent as Tier 3 are required to suppress transmission, especially considering more transmissible variants, at least until effective vaccination is widespread or much greater population immunity has amassed.

Strengths and limitations of this study

• First study to measure effects of UK Tier system for SARS-CoV-2 control at national and regional level.

• Model makes minimal assumptions and is primarily data driven.

• Insufficient statistical power to estimate effects of individual interventions that comprise Tiers, or their interaction.

• Estimates show that Tiers 1 and 2 are insufficient to suppress transmission, at least until widespread population immunity has amassed. Emergence of more transmissible variants of concern unfortunately supports this conclusion.