COVID-19 caused by either Omicron sub-variants or Delta in non-hospitalised adults associates with similar illness duration, symptom severity and viral kinetics, irrespective of vaccination history

This research has not been peer-reviewed. It is a preliminary report that should not be regarded as conclusive, guide clinical practice or health-related behaviour, or be reported in news media as established information.


SARS-CoV-2 variant Omicron rapidly evolved over 2022, causing three waves of infection due to sub-variants BA.1, BA.2 and BA.4/5. We sought to characterise symptoms and viral loads over the course of COVID-19 infection with these sub-variants in otherwise-healthy, vaccinated, non-hospitalised adults, and compared data to infections with the preceding Delta variant of concern (VOC).


In a prospective, observational cohort study, healthy vaccinated UK adults who reported a positive PCR or lateral flow test, self-swabbed on alternate days until day 10. We compared symptoms and viral load trajectories between infections caused by VOCs Delta and Omicron (sub-variants BA.1, BA.2 and BA.4/5), and tested for relationships between vaccine dose, symptoms and PCR Ct value as a proxy for viral load.


555 infection episodes were reported among 483 participants. Across VOCs, symptom burden and duration were similar, however symptom profiles differed among infections caused by Delta compared to Omicron sub-variants; symptoms of all Omicron sub-variants BA.1, BA.2 and BA.4/5 were very similar. Anosmia was reported in 7-13% of participants with Omicron sub-variants, compared to 25/60 (42%) with Delta infection (P= 1.31e-08 or 1.03e-05 or 5.63e-05; χ2 test d2+Delta vs. Omicron BA.1 or vs. BA.2, or BA.5, respectively), fever was more common with Omicron BA.5 (30/55, 55%) than Delta (20/60, 33%) (p 0.03). Amongst infections with all Omicron sub-variants, symptoms of coryza, fatigue, cough and myalgia predominated. Viral load trajectories and peaks did not differ between Delta, and Omicron, irrespective of symptom severity (including asymptomatic participants), VOC or vaccination status. Ct values were negatively associated with time since vaccination in participants infected with BA.1; however, this trend was not observed in BA.2/BA.4/5 infections.


Our study emphasises both the changing symptom profile of COVID-19 infections in the Omicron era, and ongoing transmission risk of Omicron sub-variants in vaccinated adults.

Author list


Hermaleigh Townsley1,2*, Joshua Gahir1,2*, Timothy W Russell3*, Edward J Carr1, Matala Dyke1,2, Lorin Adams4, Murad Miah1, Bobbi Clayton1, Callie Smith1, Mauro Miranda1, Harriet V Mears1, Chris Bailey1, James RM Black1,5, Ashley S Fowler1, Margaret Crawford, Katalin Wilkinson1, Matthew Hutchinson1, Ruth Harvey1,4, Nicola O’Reilly1, Gavin Kelly1, Robert Goldstone1, Rupert Beale1,5,9, Padmasayee Papineni6, Tumena Corrah6, Richard Gilson7, Simon Caidan1, Jerome Nicod1, Steve Gamblin1, George Kassiotis1,8, Vincenzo Libri2,4, Bryan Williams2,4, Sonia Gandhi1,5, Adam J Kucharski3, Charles Swanton1,5, David LV Bauer*1,9† and Emma C Wall*1,2


  1. The Francis Crick Institute, 1 Midland Road, London, UK
  2. National Institute for Health Research (NIHR) University College London Hospitals (UCLH) Biomedical Research Centre and NIHR UCLH Clinical Research Facility, London, UK
  3. Centre for Mathematical Modelling of Infectious Diseases, London School of Hygiene & Tropical Medicine, London, UK
  4. Worldwide Influenza Centre, The Francis Crick Institute, 1 Midland Road, London, UK
  5. University College London, Gower Street, London
  6. London Northwest University Healthcare NHS Trust, London
  7. Camden and North West London NHS Community Trust, London
  8. Department of Infectious Disease, St Mary’s Hospital, Imperial College London, London
  9. Genotype-to-Phenotype UK National Virology Consortium (G2P-UK)
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