Media release

From: European Congress of Clinical Microbiology & Infectious Diseases (ECCMID)

Study shows that in vaccinated patients, mean duration of COVID-19 symptoms is 2 days shorter for omicron variant (7 days) versus delta (9 days)

• Difference in symptom duration larger if 2 doses plus booster dose received - 4.4 days for omicron and 7.7 days for delta (difference 3.3 days); however, if only 2 doses received, mean symptom duration 8.3 days for omicron and 9.6 days for delta (difference 1.3 days)
• Loss of sense of smell far less common with SARS-CoV-2 omicron variant than delta, but sore throat and hoarse voice more common in omicron patients
• Patients with omicron variant 25% less likely to be admitted to hospital and also 2.5 times more likely to recover within one week than patients with delta
• If results confirmed by viral load studies, the period of infectiousness for omicron might be shorter, which would in turn impact workplace health policies and public health guidance

*Note: this paper is a special early release from the European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) and is being published in The Lancet.  Please credit both the congress and the journal in your stories*

A new study to be presented at this year’s European Congress of Clinical Microbiology & Infectious Diseases (ECCMID 2022, Lisbon 23-26), and published in The Lancet, details the different symptom patterns and duration presented by patients infected with omicron or delta variants of SARS-CoV-2. The study is by Dr Cristina Menni and Professor Tim Spector of King’s College London, UK.

The study 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 application (previously known as the COVID Symptoms Study App). The app enables self-reported information related to SARS-CoV-2 infection to be captured. Upon enrolment, users provided baseline demographic and health information. Subsequently, participants provided daily updates on symptoms experienced, SARS-CoV-2 test results, vaccines administered, and if they were self-quarantining or seeking health care, including the level of intervention and related outcomes.

Eligible participants were aged 16–99 years, based in the UK, with a BMI 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 of the study 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.

Between June 1, 2021, and Jan 17, 2022, the researchers 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, with the delta variant prevalent at >70%, n=4990; and Dec 20, 2021, to Jan 17, 2022, with the omicron variant prevalent at >70%; n=4990.

Loss of smell was less common in participants infected during omicron prevalence (17%) than during delta prevalence (53%), with statistical modelling showing those with omicron were 83% less likely to develop loss of smell. Sore throat was more common during omicron prevalence than delta, with statistical analysis showing a 55% increased risk of sore throat for omicron patients. Patients with the omicron variant were also 24% more likely to develop a hoarse voice than those with delta. Those infected during omicron prevalence were around half as likely to display at least one out of the three classic COVID-19 symptoms (fever, loss of smell, and persistent cough) compared with individuals infected with delta.

Omicron patients were also 25% less likely to admitted to hospital than delta patients (1.9% of patients admitted versus 2.6%). And the duration of acute symptoms was shorter during omicron prevalence than during delta prevalence. In a subset of participants that recovered within 21 days, the authors compared the number of days until resolution of acute symptoms in matched delta-infected and omicron-infected individuals (1530 in each group). They found that the mean duration of acute symptoms was longer for delta (mean duration 8·9 days) than for omicron (mean duration 6·9 days).

This difference appeared less marked among individuals who had received only two doses of the vaccine (delta mean duration 9·6 days, omicron mean duration 8·3 days, difference 1.3 days); and more marked in individuals who had received three doses of the vaccine (delta mean duration 7·7 days, omicron mean duration 4·4 days, difference 3.3 days).

Finally, in this same subset of individuals the authors investigated the odds of recovering within 7 days of the onset of symptoms during omicron prevalence compared with during delta prevalence. They obtained an odds ratio (adjusted for age, sex, presence of comorbidities, and vaccine doses) of 2·5, indicating that people infected during higher omicron prevalence are more 2.5 times as likely to recover within one week of the onset of symptoms than those who were infected during higher delta prevalence. On all these findings, the authors say: “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 authors note some limitations to their study that include they were unable to compare symptoms, risk of hospital admission, or duration of infection by the two variants in unvaccinated individuals, as most study participants were vaccinated. Also, infection with omicron and delta were assigned based on the prevalence in the UK population at the time and not on individual genetic sequencing from viral samples from these individuals.

The authors conclude: “We report that among vaccinated individuals, the clinical symptoms associated with symptomatic infection by the SARS-CoV-2 omicron variant are different, milder, and of shorter duration than those presented by the delta variant among vaccinated  individuals. Furthermore, loss of smell, so central in the clinical presentation of COVID-19, is much less frequently reported by those infected with omicron.”

In a linked Comment, Dr Linda Houhamdi and Dr Pierre-Edouard Fournier of The University of Aix-Marseille Université, Marseille, and Assistance Publique-Hôpitaux de Marseille, France, say: “Understanding both the characteristics of COVID-19 and the dynamics of its causative variants constitutes a crucial milestone in preventing transmission and

reducing infection, hospital admission, and death. Large-scale tracking tools that use simple and accessible devices and enable rapid and widespread analyses are extremely desirable.”

They conclude: “The severity of this disease and the unprecedented ability of SARS-CoV-2 to modify its genome and spread in successive waves highlights the usefulness of easy-to-

use mobile apps such as ZOE to rapidly assess the characteristics of a new variant and implement optimised management measures.”