Our primary outcomes were the likelihood of developing a given symptom (from the 32 monitored in the app) within 7 days before or after the positive LFAT or PCR in those infected in the omicron-dominant period compared with in those infected in the delta-dominant period, the likelihood of reporting any of the classic symptoms (fever, loss of smell, or persistent cough 10), and the likelihood of hospital admission within the disease period in the same populations. We aimed to quantify the differences in symptoms, risk of hospital admission, and duration following infection with the omicron or delta variants among people vaccinated (two or three doses) in a large community cohort from the UK drawn from the ZOE COVID Study app. The shorter presentation of symptoms suggests (pending confirmation from viral load studies) that the period of infectiousness might be shorter, which would in turn impact workplace health policies and public health guidance. The different clinical presentation is important for selection of test-triggering symptoms. However, this might not be the case in unvaccinated individuals. Our study substantiates previous suggestions that the omicron SARS-CoV-2 variant has a different clinical presentation to that of previous waves of COVID-19 in vaccinated individuals. Implications of all the available evidence Since we matched on age, sex, and number of vaccine doses, these factors are unlikely to confound our observation. We report that the symptoms characterising an omicron breakthrough infection differ from those of the delta SARS-CoV-2 variant. This is a larger, more detailed, generalisable, and less confounded study than attempted previously. However, the general presentation of symptoms compared with delta and how the duration and risk of hospitalisation vary in patients who have received two or three vaccine doses has not been reported on a large prospective population scale. A small study from Korea (n=40) showed that symptoms for omicron were mild and no patients needed supplemental oxygen. A cohort study at IHU Méditerranée Infection investigated the first 1119 omicron cases in France and reported significantly lower rates of hospitalisation, need for intensive care, or mortality compared with 3075 delta cases. Several studies from South Africa indicated that infection with the omicron variant was significantly less severe than with the previous dominant variants, with lower rates of hospital admission. We found several published papers and preprints covering differences in viral load, neutralising antibody responses among vaccinated individuals, and the description of prevalence in various regions (eg, Florida, Norway, and South Africa). We searched PubMed for articles published up to Jan 24, 2022, using the terms “SARS-CoV-2 omicron symptoms” and “SARS-CoV-2 omicron hospitalisation”. 9 However, no detailed published reports have investigated symptom prevalence and acute symptom duration, and how these compare to the delta variant. 5, 6, 7, 8 Similarly, a study 9 investigating the first 1119 omicron cases in France reported significantly lower rates of hospitalisation, need for intensive care, and mortality compared with 3075 delta cases. Hospital admission rates in South Africa for cases infected with this variant have been significantly lower than for previous waves, in which other variants of concern were dominant. 3, 4, 5, 6 A small (n=40) South Korean study 4 described the clinical presentation of omicron cases there were no severe cases. 2 Early reports suggested infection with omicron was less severe than with previous variants. 1 In the subsequent weeks, omicron spread to over 80 countries and became the dominant SARS-CoV-2 variant in the UK, overtaking the previously dominant delta variant (B.1.617.2) on Dec 20, 2021. 1 Of the many mutations detected in omicron, more than 30 are in the spike protein and 15 are in the receptor-binding domain, which could affect transmission, disease presentation, and natural or vaccine-induced protective immunity. On Nov 26, 2021, WHO designated the SARS-CoV-2 variant B.1.1.529 (omicron), first seen in South Africa, as a new variant of concern.
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