Symptom prevalence, duration, and risk of hospital admission in individuals infected with SARS-CoV-2 during periods of omicron and delta variant dominance: a prospective observational study from the ZOE COVID Study

Abstract

Background: The SARS-CoV-2 variant of concern, omicron, appears to be less severe than delta. We aim to quantify the differences in symptom prevalence, risk of hospital admission, and symptom duration among the vaccinated population.

Methods: In this prospective longitudinal observational study, we collected data from participants who were self-reporting test results and symptoms in the ZOE COVID app (previously known as the COVID Symptoms Study App). Eligible participants were aged 16-99 years, based in the UK, with a body-mass index between 15 and 55 kg/m², had received at least two doses of any SARS-CoV-2 vaccine, were symptomatic, and logged a positive symptomatic PCR or lateral flow result for SARS-CoV-2 during the study period. The primary outcome was the likelihood of developing a given symptom (of the 32 monitored in the app) or hospital admission within 7 days before or after the positive test in participants infected during omicron prevalence compared with those infected during delta prevalence.

Findings: Between June 1, 2021, and Jan 17, 2022, we identified 63 002 participants who tested positive for SARS-CoV-2 and reported symptoms in the ZOE app. These patients were matched 1:1 for age, sex, and vaccination dose, across two periods (June 1 to Nov 27, 2021, delta prevalent at >70%; n=4990, and Dec 20, 2021, to Jan 17, 2022, omicron prevalent at >70%; n=4990). Loss of smell was less common in participants infected during omicron prevalence than during delta prevalence (16·7% vs 52·7%, odds ratio [OR] 0·17; 95% CI 0·16-0·19, p<0·001). Sore throat was more common during omicron prevalence than during delta prevalence (70·5% vs 60·8%, 1·55; 1·43-1·69, p<0·001). There was a lower rate of hospital admission during omicron prevalence than during delta prevalence (1·9% vs 2·6%, OR 0·75; 95% CI 0·57-0·98, p=0·03).

Interpretation: The prevalence of symptoms that characterise an omicron infection differs from those of the delta SARS-CoV-2 variant, apparently with less involvement of the lower respiratory tract and reduced probability of hospital admission. Our data indicate a shorter period of illness and potentially of infectiousness which should impact work-health policies and public health advice.

Funding: Wellcome Trust, ZOE, National Institute for Health Research, Chronic Disease Research Foundation, National Institutes of Health, and Medical Research Council.

Figure 1

Symptoms in patients with delta or omicron SARS-CoV-2 infection (A) Prevalence of symptoms reported by individuals infected during omicron or delta prevalence. Data are percentage prevalence in the 1:1 matched sample. Error bars indicate 95% CI. (B) Association between symptoms and COVID-19 infection in 4990 participants who tested positive for SARS-CoV-2 when omicron was dominant and 4990 participants who tested positive when delta was dominant. Data are odds ratios comparing omicron and delta prevalence. Error bars indicate 95% CI.

Figure 2

Classic symptoms, hospital admission, and symptom duration in patients infected with SARS-CoV-2 during delta and omicron prevalence (A) Association between type of SARS-CoV-2 prevalence (omicron or delta) and presentation of classic symptoms (defined as at least one of loss of smell, fever, or persistent cough) in 4990 participants who tested positive for SARS-CoV-2 when omicron was dominant and 4990 participants who tested positive when delta was dominant. Error bars indicate 95% CI. (B) Self-reported hospital admission by SARS-CoV-2 variant prevalence in the same subset as (A). (C) Acute symptom duration. Proportion of participants self-reporting symptoms to the ZOE COVID Study app within 21 days after infection with SARS-CoV-2 in (i) the overall matched set, (ii) participants who received two doses of any SARS-CoV-2 vaccine, and (iii) participants who received three doses of any SARS-CoV-2 vaccine.