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Exponential growth, high prevalence of SARS-CoV-2 and vaccine effectiveness associated with Delta variant in England during May to July 2021
Paul Elliott; David J Haw; Haowei Wang; Oliver Eales; Caroline E Walters; Kylie E. C. Ainslie; Christina J Atchison; Claudio Fronterre; Peter Diggle; Andrew J Page; Alex Trotter; Sophie J Prosolek; - The COVID-19 Genomics UK (COG-UK) consortium; Deborah Ashby; Christl Donnelly; Wendy Barclay; Graham P Taylor; Graham Cooke; Helen Ward; Ara Darzi; Steven Riley.
  • Paul Elliott; Imperial College London School of Public Health
  • David J Haw; Imperial College London
  • Haowei Wang; Imperial College London
  • Oliver Eales; Imperial College London
  • Caroline E Walters; Imperial College London
  • Kylie E. C. Ainslie; Imperial College London
  • Christina J Atchison; Imperial College London
  • Claudio Fronterre; Lancaster University
  • Peter Diggle; Lancaster University
  • Andrew J Page; Quadram Institute
  • Alex Trotter; Quadram Institute Bioscience
  • Sophie J Prosolek; Quadram Institute
  • - The COVID-19 Genomics UK (COG-UK) consortium; The COVID-19 Genomics UK (COG-UK) consortium
  • Deborah Ashby; Imperial College London
  • Christl Donnelly; University of Oxford
  • Wendy Barclay; Imperial College London
  • Graham P Taylor; Imperial College London
  • Graham Cooke; Imperial College
  • Helen Ward; Imperial College London
  • Ara Darzi; Imperial College London
  • Steven Riley; Dept Inf Dis Epi, Imperial College
Preprint in English | medRxiv | ID: ppmedrxiv-21262979
BackgroundThe prevalence of SARS-CoV-2 infection continues to drive rates of illness and hospitalisations despite high levels of vaccination, with the proportion of cases caused by the Delta lineage increasing in many populations. As vaccination programs roll out globally and social distancing is relaxed, future SARS-CoV-2 trends are uncertain. MethodsWe analysed prevalence trends and their drivers using reverse transcription-polymerase chain reaction (RT-PCR) swab-positivity data from round 12 (between 20 May and 7 June 2021) and round 13 (between 24 June and 12 July 2021) of the REal-time Assessment of Community Transmission-1 (REACT-1) study, with swabs sent to non-overlapping random samples of the population ages 5 years and over in England. ResultsWe observed sustained exponential growth with an average doubling time in round 13 of 25 days (lower Credible Interval of 15 days) and an increase in average prevalence from 0.15% (0.12%, 0.18%) in round 12 to 0.63% (0.57%, 0.18%) in round 13. The rapid growth across and within rounds appears to have been driven by complete replacement of Alpha variant by Delta, and by the high prevalence in younger less-vaccinated age groups, with a nine-fold increase between rounds 12 and 13 among those aged 13 to 17 years. Prevalence among those who reported being unvaccinated was three-fold higher than those who reported being fully vaccinated. However, in round 13, 44% of infections occurred in fully vaccinated individuals, reflecting imperfect vaccine effectiveness against infection despite high overall levels of vaccination. Using self-reported vaccination status, we estimated adjusted vaccine effectiveness against infection in round 13 of 49% (22%, 67%) among participants aged 18 to 64 years, which rose to 58% (33%, 73%) when considering only strong positives (Cycle threshold [Ct] values < 27); also, we estimated adjusted vaccine effectiveness against symptomatic infection of 59% (23%, 78%), with any one of three common COVID-19 symptoms reported in the month prior to swabbing. Sex (round 13 only), ethnicity, household size and local levels of deprivation jointly contributed to the risk of higher prevalence of swab-positivity. DiscussionFrom end May to beginning July 2021 in England, where there has been a highly successful vaccination campaign with high vaccine uptake, infections were increasing exponentially driven by the Delta variant and high infection prevalence among younger, unvaccinated individuals despite double vaccination continuing to effectively reduce transmission. Although slower growth or declining prevalence may be observed during the summer in the northern hemisphere, increased mixing during the autumn in the presence of the Delta variant may lead to renewed growth, even at high levels of vaccination.
Full text: Available Collection: Preprints Database: medRxiv Document Type: Preprint Language: English Year: 2021





Full text: Available Collection: Preprints Database: medRxiv Document Type: Preprint Language: English Year: 2021