Differences in the case fatality risks associated with SARS-CoV-2 Delta and non-Delta variants in relation to vaccine coverage: An early ecological study in the United Kingdom

Abstract

The circulation of SARS-CoV-2 Delta (i.e., B.1.617.2) variants challenges the pandemic control. Our analysis showed that in the United Kingdom (UK), the reported case fatality ratio (CFR) decreased from May to July 2021 for non-Delta variant, whereas the decreasing trends of the CFR of Delta variant appeared weak and insignificant. The association between vaccine coverage and CFR might be stratified by different circulating variants. Due to the limitation of ecological study design, the interpretation of our results should be treated with caution.

Keywords: COVID-19; Case fatality risk; Delta variants; United Kingdom; Vaccine.

Fig. 1

The COVID-19 cases and deaths, Delta variants, case fatality ratios, and vaccine coverage in the UK. The daily number of COVID-19 cases and deaths (A), proportion of the SARS-CoV-2 Delta variants (B), the reconstructed instantaneous (or time-varying) case fatality ratios (CFR) (D), and vaccine coverage (D) in the United Kingdom (UK). In panel (B), the sample size of SARS-CoV-2 strains collected on each day (denoted by n) is reflected by the size of each dot. Panel (C) shows that the estimated CFRs of non-Delta (blue), and Delta (red) variants. In panel (D), the green curve is the vaccine coverage, and the orange bars and area are the estimated relative ratios (RR) of CFR of Delta against that of non-Delta variants. In panels (C) and (D), the horizontal bars are the maximal likelihood estimates (MLE), and the shading areas indicate the 95% confidence intervals (95%CI). Panels (E) and (F) show that the statistical relation between vaccine coverage and CFR of non-Delta (triangular) and Delta (diamond) variants, respectively. In panel (E), the circled dots at the top-right corner are the CFR of Alpha variants. In panels (E) and (F), the dashed lines are the estimated linear associations between vaccine coverage (lagged for +21 days accounting for the delay in the effects of immune responses) and CFR, where the statistical significances are presented in the panel legends. The dots and bars are the MLEs and 95%CIs, respectively. Note: The contents in panels (E) and (F) are directly extracted from the CFR estimates and vaccine coverage observations in panels (C) and (D) by matching the calendar dates. The scales of axes in panels (E) and (F) are the same. From April 1 to July 15, 2021, more than 99.9% out of a total of over 150 thousand SARS-CoV-2 strain samples in the UK are either Delta or Alpha (i.e., B.1.1.7) variants, and thus non-Delta variants are (almost) equivalent to the Alpha variants in the situation of this study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)