1. Main points
The risk of death involving coronavirus (COVID-19) is 67% lower following an Omicron infection compared with a Delta infection.
The difference in the risk of death involving COVID-19 between Omicron and Delta varied by age, where the reduction in risk was greater among those aged 18 to 59 years (87%) and those aged 60 to 69 years (86%), than among those aged 70+ (55%).
The difference in the risk of death involving COVID-19 between Omicron and Delta varied by sex, with males having a greater reduction in risk of death than females (75% and 56% respectively).
The World Health Organization (WHO) defines coronaviruses as "a large family of viruses that are known to cause illness ranging from the common cold to more severe diseases such as Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS)".
COVID-19 refers to the "coronavirus disease 2019" and is a disease that can affect the lungs and airways. It is caused by a type of coronavirus. Further information is available from the World Health Organization (WHO).
Cox proportional hazards regression model
The Cox proportional hazards regression model is a multiple regression procedure that measures the association between a time-to-event outcome and a characteristic of interest such as age, while adjusting for other characteristics expected to also be associated with the outcome.
95% confidence intervals
A confidence interval is a measure of the uncertainty around a specific estimate. If a confidence interval is 95%, it is expected that the interval will contain the true value on 95 occasions if repeated 100 times. As intervals around estimates widen, the level of uncertainty about where the true value lies increases. The size of the interval around the estimate is strongly related to the number of deaths, prevalence of health states and the size of the underlying population. At a national level, the overall level of error will be small compared with the error associated with a local area or a specific age and sex breakdown. More information is available on our uncertainty pages.
Deaths involving COVID-19
For this analysis we define a death as involving COVID-19 if the International Classification of Diseases 10th edition (ICD-10) code U07.1 (COVID-19, virus identified) is mentioned on the death certificate.
A hazard ratio is a measure of the relative differences in the instantaneous rate of mortality between groups. A hazard ratio greater than one indicates the rate of mortality is higher, and likewise less than one lower, in the population group under study compared with a reference group.Back to table of contents
6. Data sources and quality
Our analysis is based on the Office for National Statistics (ONS) Public Health Data Asset (PHDA), linked to national testing data taken in the community from NHS Test and Trace (pillar 2). The Public Health Data Asset (PHDA) is a linked dataset combining the 2011 Census, the General Practice Extraction Service (GPES) data for coronavirus (COVID-19) pandemic planning and research, the Hospital Episode Statistics (HES), death registration data and COVID-19 vaccination data from the National Immunisation Management Service (NIMS). The data were restricted to PCR positive tests taken in England between 1 and 31 December 2021 in the community and analysed by Lighthouse laboratories.
Provisional death registrations up until 25 January 2022 have been used to identify deaths involving COVID-19, which refers to deaths that had COVID-19 mentioned anywhere on the death certificate.
The COVID-19 variant was ascertained using S-gene target failure (SGTF). Tests were identified Omicron-compatible if S-negative, N-positive, ORF1ab-positive (with mean Ct less than 30) or Delta-compatible if S-positive, N-positive, ORF1ab-positive or ORF1ab-positive, S-positive or N-positive, S-positive, and mean Ct less than 30.
We used cause-specific Cox proportional hazard regression models to estimate the hazard ratio of COVID-19 related death for individuals infected with Omicron versus Delta variants. For non-COVID-19 deaths, individuals were censored at the date of death if this occurred before the end of the study date. We adjusted for sex, age, vaccination status, previous COVID-19 infection, calendar time, ethnicity, Index of Multiple Deprivation rank, household deprivation, university degree, keyworker status, country of birth, main language, region, disability, and the number of pre-existing conditions (as defined in the Qcovid 3 risk model). To test whether the relative risk of mortality of Omicron varied by specific characteristics, we included interactions between variant type and age, and variant type and sex. In addition, we fitted separate fully adjusted models for those aged 18 to 59 years, those aged 60 to 69 years, and those aged 70 years and over to look at interactions between variant and the number of health conditions (zero, one to two, or three and more).
We used a large sample of positive cases from the national testing programme, allowing to precisely estimate the relative risk of COVID-19 death following infection with Omicron and Delta variants. We used death certificate data to confirm COVID-19 mortality, preventing our sample being conflated with non-COVID-19 related deaths of individuals that die of other causes following a positive COVID-19 test. We also compared the outcomes during the same time period overcoming any differences because of changes in management of infected patients over the time period of the pandemic. We also adjusted for a wide range of potential confounding factors.
Provisional death registrations and death occurrences data are used in this article. This enables timely analysis to be completed to monitor mortality trends. However, as the data for 2021 are provisional, they are subject to change. This article uses death occurrences registered up to 25 January 2022. Because of registration delays, more deaths may be registered at later dates, leading to an increase in the death occurrences. This is especially true for more recent deaths. The data are for England only, as vaccinations data for Wales are not yet available and the Public Health Data Asset (PHDA) covers England only.Back to table of contents
7. Future developments
Impact of coronavirus (COVID-19) variants on health outcomes
Importantly, our results indicate that the risk of a death involving coronavirus (COVID-19) following infection with Omicron relative to Delta is significantly lower, however mortality is only one metric to measure the impact of infection. Subsequent work will investigate long-term outcomes of infection, such as the prevalence of long COVID following Omicron infection.Back to table of contents
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