Comparative transmission of SARS-CoV-2 Omicron (B.1.1.529) and Delta (B.1.617.2) variants and the impact of vaccination: national cohort study, England

  1. Hester Allen
  2. Elise Tessier
  3. Charlie Turner
  4. Charlotte Anderson
  5. Paula Blomquist
  6. David Simons
  7. Alessandra Løchen
  8. Christopher I Jarvis
  9. Natalie Groves
  10. Fernando Capelastegui
  11. Joe Flannagan
  12. Asad Zaidi
  13. Cong Chen
  14. Christopher Rawlinson
  15. Gareth J. Hughes
  16. Dimple Chudasama
  17. Sophie Nash
  18. Simon Thelwall
  19. Jamie Lopez-Bernal
  20. Gavin Dabrera
  21. André Charlett
  22. Meaghan Kall
  23. Theresa Lamagni
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Abstract

Background

The SARS-CoV-2 Omicron variant (B.1.1.529) has rapidly replaced the Delta variant (B.1.617.2) to become dominant in England. This epidemiological study assessed differences in transmissibility between the Omicron and Delta using two methods and data sources.

Methods

Omicron and Delta cases were identified through genomic sequencing, genotyping and S-gene target failure in England from 5-11 December 2021. Secondary attack rates for Omicron and Delta using named contacts and household clustering were calculated using national surveillance and contact tracing data. We used multivariable logistic regression was used to control for factors associated with transmission.

Findings

Analysis of contact tracing data identified elevated secondary attack rates for Omicron vs Delta in household (15.0% vs 10.8%) and non-household (8.2% vs 3.7%) settings. The proportion of index cases resulting in residential clustering was twice as high for Omicron (16.1%) compared to Delta (7.3%). Transmission was significantly less likely from cases, or in named contacts, in receipt of three compared to two vaccine doses in household settings, but less pronounced for Omicron (aRR 0.78 and 0.88) compared to Delta (aRR 0.62 and 0.68). In non-household settings, a similar reduction was observed for Delta cases and contacts (aRR 0.84 and 0.51) but only for Omicron contacts (aRR 0.76, 95% CI: 0.58-0.93) and not cases in receipt of three vs two doses (aRR 0.95, 0.77-1.16).

Interpretation

Our study identified increased risk of onward transmission of Omicron, consistent with its successful global displacement of Delta. We identified a reduced effectiveness of vaccination in lowering risk of transmission, a likely contributor for the rapid propagation of Omicron.

Funding

Study funded by the UK Health Security Agency.

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