People with the Omicron variant of SARS-CoV-2 were less likely to be admitted to hospital or to die, compared to those with the Delta variant.
After adjusting for a number of factors, the risk of hospital admission for Omicron cases was found to be less than half (59% lower) compared to the risk for Delta cases. The risk of dying was 69% lower for those with Omicron compared to those with Delta infections. The research, published in The Lancet, was led by researchers at the MRC Centre for Global Infectious Disease Analysis, Jameel Institute at Imperial College London, the MRC Biostatistics Unit at the University of Cambridge, and the UK Health Security Agency.
Severity and age
The difference in risk between Omicron and Delta varied considerably with age. The risk of hospitalisation differed the most for those aged 60-69 years (75% lower for Omicron compared with Delta). In contrast, for children under the age of 10 years, there was no significant difference in risk of hospitalisation between Omicron and Delta. The risk of death remained very low for children, regardless of which SARS-CoV-2 variant they were infected with. It is possible that the similarity between variants in hospitalisation risk for children is due to precautionary admissions, given the different symptom profile of Omicron compared to Delta.
Protection by vaccination or previous infection
"Vaccines continue to offer high levels of protection against the Omicron variant. Make sure that you come forward for vaccination when eligible and take up the offer of a booster dose as soon as possible." Dr Susan Hopkins, UK Health Security Agency
Vaccinated individuals were at considerably lower risks of both hospital admission and death compared to unvaccinated individuals, whether infected with Omicron or Delta. For those who had received three vaccine doses before their positive test, the risk of hospital admission was approximately 80% lower, and the risk of death approximately 85% lower compared to unvaccinated cases. Among unvaccinated cases, for whom the risks were higher than for vaccinated cases, the risk of hospital admission was 70% lower and the risk of death 80% lower if infected with Omicron, compared to unvaccinated cases infected with Delta. This finding indicates that the Omicron variant has a lower intrinsic severity than the Delta variant.
Having had COVID-19 previously also offered some protection, likely due to the immunity from a past infection: the risk of hospital admission was 45% lower for unvaccinated cases who had had a known past infection, compared to unvaccinated cases for whom the infection was their first. For vaccinated cases with past infection, there appeared to be no additional reduction in the already low risk of hospitalisation over and above the protection given by vaccination. The risk of death was, however, lower for those with past infection in both unvaccinated and vaccinated individuals.
Largest study on severity
"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. Prof Neil Ferguson, Imperial College London
This is the largest study to date to report on the risk of hospitalisation and mortality outcomes for individuals with the Omicron compared to the Delta variant, using data from 1,516,702 Omicron and Delta cases who tested positive for COVID-19 between 29th November 2021 and 9th January 2022. The Omicron variant has previously been shown to spread faster than the Delta variant and Omicron 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 to the Delta variant is therefore reassuring in terms of the potential hospital burden and number of lives lost from Omicron variant epidemics, particularly in populations with high immunity due to vaccination. However, it is important to note that the lower relative severity is compared to the Delta variant, which was associated with higher risks of severe disease than Alpha, which was in turn more severe than the earliest circulating variants. Although the risks associated with Omicron are most likely lower than the risks with Delta, this does not directly translate to a low risk in general, particularly for unvaccinated individuals with no past infection.
Previous studies have shown that although both Delta and Omicron may break through the immunity from vaccination and result in infection, the available COVID-19 vaccines are effective at preventing symptoms and severe disease for both the Delta and Omicron variants, particularly after receiving a third booster dose. In line with these results from previous vaccine effectiveness studies, the researchers found that those who became infected despite vaccination had lower risks of admission and death compared to those unvaccinated.
Professor Neil Ferguson, Imperial College London, 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.
Dr Anne Presanis, 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. But these results should be interpreted in the context that Delta is more severe than previous variants, and Omicron is not necessarily mild, particularly for those unvaccinated. Coupled with the high transmissibility of Omicron, hospital burden and mortality could be high in populations with low vaccination coverage. Getting three vaccine doses is therefore crucial for reducing the burden on healthcare systems."
Dr Susan Hopkins, UK Health Security Agency, said: "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.
"Vaccines continue to offer high levels of protection against the Omicron variant. Make sure that you come forward for vaccination when eligible and take up the offer of a booster dose as soon as possible."
