1. Loss of sense of smell was more common during a period of delta dominance, while sore throat was more common during a period of omicron dominance.
2. Hospitalizations were more common among COVID-positive patients during the period of delta variant dominance.
Evidence Rating Level: 2 (Good)
Study Rundown: Two major variants of COVID-19 struck the UK at two distinct time periods. However, few studies have compared the differences in symptomatology between COVID-19 infections of different variants. This study aimed to characterize the differences between the two variants, Delta (B.1.617.2) and Omicron (B.1.1.529), by using a self-reporting application for participants. A total of 63002 app users tested positive and submitted symptoms. After separating participants depending on whether delta or omicron was more prevalent when they tested positive, participants were matched 1:1 based on age, sex and vaccination dose. Loss of sense of smell was significantly more common in the delta group while sore throat was more common in the omicron group. Hospitalization was also more prevalent in the delta group, with a greater median number of symptoms overall. Limitations of this study include the self-reported nature of the data and the lack of confirmation of the variant each participant had. Nonetheless, this study demonstrated that less severe symptoms were present during the omicron wave compared to the delta wave in the UK. It also successfully employed an application to track symptoms which could be used for future variants.
Relevant Reading: Increased immune escape of the new SARS-CoV-2 variant of concern Omicron
In-Depth [prospective observational study]: The ZOE COVID Study collected and analyzed self-reported health information via app. Eligibility criteria included participants aged 16-99 years and who had received two doses of any COVID vaccine. Patients needed to be symptomatic and required a positive PCR or LFAT test. A total of 63,002 application users tested positive and reported symptoms. The study used two distinct time periods of either delta prevalence (June 1 to Nov 27, 2021) or omicron prevalence (Dec 20, 2021, to Jan 17, 2022). Patients in each variant group were matched 1:1 for age, sex, and vaccination dose. After 1:1 matching, the number of participants in each variant group was 4990.
The most frequent symptoms reported during delta dominance were runny nose (81.6%), headache (77.9%), sneezing (70.7%), sore throat (60.8%), and loss of smell (52.7%). The most frequent symptoms reported during omicron dominance were runny nose (76.5%), headache (74.7%), sore throat (70.5%), sneezing (63.0%), persistent cough (49.8%), and hoarse voice (42.6%). There was a greater median number of symptoms in the delta prevalent group. The study calculated odds ratios for each symptom for the omicron versus delta group. Significant differences were seen in loss of smell (OR 0.17, 95% CI 0.16-0.19, p<0.001) and sore throat (OR 1.55, 1.43-1.69, p<0.001). The hospital admission rate was also less for omicron versus delta (OR 0.75, 0.57-0.98, p=0.03). Overall, patients infected during the omicron-dominant period were less likely to report at least one of the classical COVID-19 symptoms of fever, persistent cough, and loss of smell, compared to those infected during delta-dominant periods (OR 0.56, 95% CI 0.51-0.61; p<0.001).
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