Exponential growth, high prevalence of SARS-CoV-2, and vaccine effectiveness associated with the Delta variant

Paul Elliott, David Haw, Haowei Wang, Oliver Eales, Caroline E. Walters, Kylie E. C. Ainslie, Christina Atchison, Claudio Fronterre, Peter J. Diggle, Andrew J. Page, Alexander J. Trotter, Sophie J. Prosolek, The COVID-19 Genomics UK (COG-UK) Consortium, Deborah Ashby, Christl A. Donnelly, Wendy Barclay, Graham Taylor, Graham Cooke, Helen Ward, Ara DarziSteven Riley

Research output: Contribution to journalArticlepeer-review

75 Citations (Scopus)
6 Downloads (Pure)


Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections were rising during early summer 2021 in many countries as a result of the Delta variant. We assessed reverse transcription polymerase chain reaction swab positivity in the Real-time Assessment of Community Transmission-1 (REACT-1) study in England. During June and July 2021, we observed sustained exponential growth with an average doubling time of 25 days, driven by complete replacement of the Alpha variant by Delta and by high prevalence at younger, less-vaccinated ages. Prevalence among unvaccinated people [1.21% (95% credible interval 1.03%, 1.41%)] was three times that among double-vaccinated eople [0.40%(95% credible interval 0.34%, 0.48%]. However, after adjusting for age and other variables, vaccine effectiveness for double-vaccinated people was estimated at between ∼50% and ∼60% during this period in England. Increased socialmixing in the presence of Delta had the potential to generate sustained growth in infections, even at high levels of vaccination.

Original languageEnglish
Article numberabl9551
Issue number6574
Publication statusPublished - 17 Dec 2021

Cite this