Evaluation of the impact of tiered restrictions introduced in England during October and December, 2020, on COVID-19 cases: a synthetic control study

BACKGROUND: In 2020, a second wave of COVID-19 cases unevenly affected places in England, and different levels of tiered restrictions were introduced in different parts of the country. Previous research has examined the impact of national lockdowns on transmission. We aimed to examine the differences in the effect of localised restrictions on COVID-19 cases and how these differences varied by deprivation. METHODS: We examined the transmission impact of tier 3 restrictions using data on weekly reported COVID-19 cases, adjusted for case-detection rates for 7201 neighbourhoods in England. We identified areas that entered tier 3 restrictions in October and December, 2020, constructed a synthetic control group of places under tier 2 restrictions, and compared changes in weekly infections over a 4-week period. Sufficiently granular data on deaths were not available to investigate excess mortality. We analysed whether this effect varied by level of deprivation and the prevalence of a new variant (B.1.1.7), by stratifying the synthetic control weighting by subgroups and then including an interaction term between subgroup and intervention in the regression model. We used the English Indices of Multiple Deprivation and its tertiles in the stratification to measure deprivation. We tested the spatial spillover effects excluding tier 2 areas within 20 km of tier 3 areas. Ethics approval was not needed. FINDINGS: The introduction of tier 3 restrictions was associated with a reduction in infections of 14% (95% CI 10–19) in October and of 20% (13–29) in December, or with a reduction in absolute number of total infections of 3536 (95% CI 2880–4192) in October and of 92 732 (49 776–135 688) in December, compared with what would have been expected under tier 2 restrictions. The effects were similar across levels of deprivation and by the prevalence of the new variant. We found smaller effects with high non-significant p values when excluding boundary areas. INTERPRETATION: Compared to tier 2 restrictions, restrictions on hospitality and meeting outdoors in tier 3 areas had a moderate effect on transmission and these restrictions did not appear to increase inequalities in COVID-19 cases. Limitations include a lack of specificity as to which of the main restrictions contributed to this effect, potentially biases from the crude measure of case-detection rates applied, and the lack of controls for individual or household characteristics in this ecological analysis. FUNDING: National Institute for Health Research (NIHR).