Efficacy of vaccination against severe COVID-19 in relation to Delta variant and time since second dose: the REACT-SCOT case-control study

Abstract

Background - The objectives of this study were to investigate whether vaccine efficacy against severe COVID-19 has decreased since Delta became the predominant variant; (2) whether efficacy wanes with time since second dose. Methods Efficacy was estimated in a matched case-control study that includes all diagnosed cases of COVID-19 in Scotland up to 8 September 2021. The main outcome measure was severe COVID-19, defined as cases with entry to critical care or fatal outcome.

Findings - Efficacy of vaccination against severe COVID-19 decreased in May 2021 coinciding with the replacement of the B.1.1.7 (Alpha) by the B.1.617.2 (Delta) variant in Scotland, but this decrease was reversed over the next month. In the most recent time window, the efficacy of two doses was 91% (95 percent CI 87% to 94%) for the AstraZeneca product and 92% (95 percent CI 88% to 95%) for mRNA (Pfizer or Moderna) products. Efficacy of the AstraZeneca product against severe COVID-19 declined with time since second dose to 69% (95 percent CI 52% to 80%) at 20 weeks from second dose. Efficacy of mRNA vaccines declined in the first ten weeks from second dose but more slowly thereafter to 93% (95 percent CI 88% to 96%) at 20 weeks from second dose.

Interpretation - These results are reassuring with respect to concerns that efficacy against severe COVID-19 might have fallen since the Delta variant became predominant, or that efficacy of mRNA vaccines wanes with increasing time since second dose. However it is now clear that efficacy of the AstraZeneca vaccine against severe COVID-19 wanes substantially by 20 weeks from second dose. The basis for recommending booster doses in relatively low-risk individuals will depend upon the extent to which these doses protect against new variants.

Funding - No specific funding was received for this work. HC is supported by an endowed chair from the AXA Research Foundation.

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https://researchonline.gcu.ac.uk/en/publications/6c974b6c-86c4-41a0-a4e5-ea36dd6eadb0