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

ObjectiveMeasure 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. DesignModelling study combining estimates of the real-time reproduction number Rt (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. SettingThe UK at Lower Tier Local Authority (LTLA) level. Primary and secondary outcome measuresReduction in real-time reproduction number Rt. ResultsNationally, transmission increased between July and late September, regional differences notwithstanding. Immediately prior to the introduction of the tier system, Rt 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. ConclusionsThe 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 studyO_LIFirst study to measure effects of UK Tier system for SARS-CoV-2 control at national and regional level. C_LIO_LIModel makes minimal assumptions and is primarily data driven. C_LIO_LIInsufficient statistical power to estimate effects of individual interventions that comprise Tiers, or their interaction. C_LIO_LIEstimates 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. C_LI