The Omicron variant of SARS-CoV-2 is associated with significantly lower risks of hospitalisation and mortality compared with the Delta variant, according to the largest study to date to directly compare outcomes.
Researchers in a multicentre UK team led by Imperial College London, the MRC Biostatistics Unit at the University of Cambridge, the MRC Centre for Global Infectious Disease Analysis, and the UK Health Security Agency (UKHSA) used data from 1,516,702 people in England who tested positive for SARS-CoV-2 between 29th November 2021 and 9th January 2022, and for whom whole-genome sequencing or genotyping had been performed and shown laboratory-confirmed or likely Omicron or Delta variants.
The cohort represented over a third of all positive test samples in England over the period. Test results were linked to routine datasets on vaccination status, hospital attendance and admission, and mortality.
Omicron Transmission Faster Than Delta
After adjusting for multiple potential confounding factors such as age, sex, ethnicity, and deprivation index, the risk of hospital admission for Omicron was found to be 59% lower, and the risk of dying 69% lower for individuals with Omicron compared with those with Delta infections. Risk profiles varied considerably with age, and both vaccination and previous infection conferred protection against hospitalisation and mortality.
The authors said that the Omicron variant has been shown to spread faster than the Delta variant, and is now the most common SARS-CoV-2 lineage on all continents. It currently causes more than 99% of new COVID-19 diagnoses in England, and more than 80% globally. "The evidence provided in this study on lower severity with the Omicron variant compared with the Delta variant is therefore reassuring," they say.
Dr Susan Hopkins, chief medical advisor at the UKHSA, who was not directly involved in the study, commented: "This research supports our understanding that the risk of hospitalisation following SARS-CoV-2 infection is substantially lower for Omicron compared with Delta cases. It also confirms that the risk of hospitalisation for children with either Omicron or Delta is very low."
The research, published in The Lancet, showed that the overall risk of hospitalisation within the 14 days after a positive test was 1.64% with Delta and 0.9% with Omicron. Overall mortality risk in the 28 days after a positive test was 0.27% after Delta, falling to 0.11% after Omicron. The difference in risk varied considerably with age, being 75% lower for Omicron compared with Delta for those aged 60-69 years.
No Significant Difference Shown in Children
In contrast, for children aged 10 and under, there was no significant difference in risk of hospitalisation between Delta and Omicron, and the risk of death remained very low regardless of the variant. The team commented, however, that given the different symptom profile of Omicron compared with Delta, it is possible that the similarity between variants in hospitalisation risk for children was due to precautionary admissions.
Lower Risk of Death With Omicron Than Delta in Unvaccinated Population
Among unvaccinated people, the risk of hospital admission was 70% lower and the risk of death 80% lower with Omicron compared with Delta. "This finding indicates that the Omicron variant has a lower intrinsic severity than the Delta variant," the authors said.
In line with these results from previous vaccine effectiveness studies, the researchers found that those who became infected despite vaccination had lower risks of both hospital admission and death compared with those unvaccinated. Across the sample, this was the case for both Omicron and Delta. For those who tested positive after 3 vaccine doses, the risk of hospital admission was approximately 80% lower, and the risk of death approximately 85% lower, compared with unvaccinated individuals.
Previous COVID-19 infection also offered protection among unvaccinated individuals, who had a 45% lower risk of hospital admission than unvaccinated people who had not been previously infected. Among vaccinated people, past infection appeared not to confer an additional reduction in the risk of hospitalisation. However, in both unvaccinated and vaccinated individuals, the risk of death was lower among those who had been infected in the past.
Vaccination Can Be Beneficial in Reducing Risk With Both Variants
Co-author Dr Anne Presanis PhD, senior statistician at the MRC Biostatistics Unit, University of Cambridge, said: "Our analysis highlights both that Omicron is less severe than Delta, and that vaccination substantially reduces severe disease for both variants."
Prof Neil Ferguson, DPhil, professor of mathematical biology and director of the MRC Centre for Global Infectious Disease Analysis, Jameel Institute, Imperial College London, and also a co-author, said: "This collaboration between Imperial College, Cambridge University, and UKHSA has generated the most detailed insight yet into the severity of infections caused by the Omicron variant. We found that Omicron is 60% less likely to result in hospital admission than Delta, and 70% less likely to result in death.
"Although we find that vaccines are somewhat less effective at preventing hospitalisation in Omicron cases than for Delta, their effectiveness is still substantial. The paper shows that vaccination is always highly beneficial in reducing risk, even for Omicron.
"Interestingly, how much severity is reduced varies by age, with the greatest reduction in severity seen in 50-70 year olds and a smaller reduction in younger and older age groups.
"While we cannot be sure that future variants will retain the characteristics of Omicron, the large reduction in severity seen has undoubtedly made it easier for countries to end pandemic restrictions than might otherwise have been the case."
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