The omicron variant of SARS-CoV-2 (PANGO B. 1.1. 529) spread rapidly across the world, out-competing former variants soon after it was first detected in November, 2021. According to the Our World in Data COVID-19 database, In Europe, the number of confirmed cases reported between December, 2021, and March, 2022 (omicron period) has exceeded all previously reported cases. Omicron appears to cause less severe acute illness than previous variants, at least in vaccinated populations. However, the potential for large numbers of people to experience longterm symptoms is a major concern, and health and workforce planners need information urgently to appropriately scale resource allocation. In this case-control observational study, we set out to identify the relative odds of long-COVID (defined following the National Institute for Health and Care Excellence guidelines as having new or ongoing symptoms 4 weeks or more after the start of acute COVID-19) in the UK during the omicron period compared with the delta period. We used self-reported data from the COVID

Symptom Study app1 (King’s College London Research Ethics Management Application System number 18210, reference LRS-19/20-18210). Data were extracted and pre-processed using ExeTera13 (version 0.5. 5). The inclusion criteria in both periods were a positive real-time PCR or lateral flow antigen test for SARS-CoV-2 after vaccination, at least one log per week in the app for at least 28 days after testing positive, 2 and no previous SARS-CoV-2 infections before vaccination. We identified 56 003 UK adults first testing positive between Dec 20, 2021, and March 9, 2022, who satisfied the inclusion criteria. These cases are hereafter referred to as omicron cases as more than 70% of UK cases were estimated to be attributable to the omicron variant during that time. Using identical selection criteria, we identified 41 361 UK adult cases first testing positive between June 1, 2021, and Nov 27, 2021, referred to as delta cases as more than 70% of cases were attributable to the delta variant. Both symptomatic and asymptomatic infections were considered, and, for the omicron period, we included only participants testing positive before Feb 10, 2022, to ensure all participants had at least 28 days for symptom reporting after testing positive. In both periods, female participation was higher than male participation