Characterization of the severity of Omicron relative to Delta in England

In a recent study posted to Preprints with The Lancet*, researchers assessed the relative risks of hospitalization and death by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant compared to those by Delta (B.1.617.2). In addition, they stratified the magnitude of risk by age and provided estimates of how immunity from both vaccination and past infection modified disease severity in breakthrough cases.

Study: Comparative Analysis of the Risks of Hospitalisation and Death Associated with SARS-CoV-2 Omicron (B.1.1.529) and Delta (B.1.617.2) Variants in England. Image Credit: Dmitry Demidovich/ShutterstockStudy: Comparative Analysis of the Risks of Hospitalisation and Death Associated with SARS-CoV-2 Omicron (B.1.1.529) and Delta (B.1.617.2) Variants in England. Image Credit: Dmitry Demidovich/Shutterstock

In England, the first Omicron case was reported on 27 November 2021. The number of confirmed Omicron cases rapidly increased, and by 10 January 2022, Omicron cases represented >99% of all sequenced cases, replacing Delta as the dominant variant in England.

Although several studies have indicated that the clinical severity of coronavirus disease 2019 (COVID-19) infection is lower for Omicron than Delta. However, characterization of age variation in the severity of Omicron infection and how reduced severity and immunity have shaped hospitalizations and deaths patterns in the Omicron wave is missing.

About the study

In the present study, researchers conducted a retrospective cohort study from 29 November 2021 to 9 January 2022; during the study period, although there were 4,135,042 COVID-19 cases in England, only 37% had available variant classification data and met the criteria to be included in the analysis. 

The researchers examined cases where patients attended the hospital within 14 days after a positive test or died within 28 days following confirmed infection. They used the stratified cox proportional hazards regression model to estimate hazard ratios (HRs) for the hospitalization and mortality outcomes. 

The study model stratified specimen by test date, 10-year age band, region, ethnicity group, and vaccination status for the primary analysis. Further regression adjustments included sex, index of socioeconomic deprivation, evidence of a previous SARS-CoV-2 infection, and the exact age within each age band. 

In the secondary analysis, the researchers estimated the overall Omicron: Delta HRs for hospitalization and death based on the intrinsic relative severity of Omicron infection, varying with variant and vaccine-specificity.

Sensitivity analyses allowed researchers to examine the impact of finer age stratification and the interaction of past infection status with the Omicron variant.

Study findings

The results revealed 448,797 Delta and 1,067,736 Omicron cases, with 1.3% of Delta and 9.6% of Omicron cases being reinfections, wherein an individual tested SARS-CoV-2-positive twice within 90 days. Additionally, they observed a steady increase in the proportion of Omicron cases each week during December 2021. Compared to Delta cases, these Omicron cases were among people from the Black community or those who resided in London or the North-West of England.

The researchers observed that compared to Delta cases, the risk of hospital admission, any hospital attendance, and death was 59%, 44%, and 69% lower for Omicron cases, respectively. Moreover, the magnitude of this risk reduction was age-dependent. Therefore, the authors noted that in the age group of 0–9-years, the risk of hospitalization from Omicron infection was not significantly different from Delta infection; however, the risk of hospitalization was much lower for Omicron cases in individuals over 20 years.

In Omicron-infected individuals over 50 years, the reduction in the risk of hospitalization was in the range of 50-75%, varying with the hospital outcome examined. Although lower, the magnitude of severity reduction was still over 50% for most endpoints for those over 80 years. The Omicron: Delta HRs in age groups <1, 1-4, and 5-9 were statistically insignificant.

Among unvaccinated Omicron cases, prior SARS-CoV-2 infection provided moderate protection against hospitalization and higher protection against death. On the other hand, among vaccinated Omicron cases, although previous SARS-CoV-2 infection offered no additional protection against hospitalization, it moderately protected against death.

In breakthrough cases, although vaccination provided reduced protection against hospitalization in Omicron versus Delta infections, mRNA booster vaccination was found to be highly protective, offering over 70% protection against hospitalization and mortality in Omicron breakthrough cases. This finding was consistent with the results of studies solely examining vaccine effectiveness.


According to the authors, the current study is among the few studies demonstrating how the reduced severity of Omicron cases varies with age. Additionally, the findings showed an overall reduction of five-fold in the intrinsic risk of death by Omicron infection compared with Delta. To conclude, these findings make “living with COVID” substantially easier, provided emerging SARS-CoV-2 variants show similarly reduced severity.

*Important notice

Preprints with The Lancet publish preliminary scientific reports that are not peer-reviewed and,
therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Neha Mathur

Written by

Neha Mathur

Neha is a digital marketing professional based in Gurugram, India. She has a Master’s degree from the University of Rajasthan with a specialization in Biotechnology in 2008. She has experience in pre-clinical research as part of her research project in The Department of Toxicology at the prestigious Central Drug Research Institute (CDRI), Lucknow, India. She also holds a certification in C++ programming.


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