The Omicron variant had higher transmissibility than the original virus that causes COVID-19. Studies indicated lower severity of infection than that of the delta variant.
Considering several factors, a new study aimed to better characterize omicron severity relative to Delta by assessing the relative risk of hospital attendance, hospital admission, or death in a large national cohort. The study involved scientists from 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.
Scientists found that the risk of hospitalization for Omicron was less than half (59% lower) compared to the risk for Delta cases. The risk of dying was 69% lower than those with Delta infections.
The severity of the virus varied with age. Those aged 60-69 years had a 75% lower risk of hospitalization for Omicron than Delta. No significant difference was observed in the risk of hospitalization for children under the age of 10 years. The risk of death remained very low for children, regardless of which SARS-CoV-2 variant they were infected with.
Those vaccinated had significantly lower risks of both hospital admission and death compared to unvaccinated individuals, whether infected with Omicron or Delta. Vaccinated individuals (who had received the vaccine before their positive test) had an 80% lower risk of hospitalization and 85% lower than 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 than unvaccinated cases infected with Delta.
Those infected with COVID-19 twice had a lower risk for hospitalization and death. It may be due to the immunity from a past infection. The risk of hospital admission was 45% lower for unvaccinated cases. There appeared to be no additional reduction in the already low risk of hospitalization over and above the protection given by vaccination for vaccinated cases. However, the risk of death was lower for those with past infection in both unvaccinated and vaccinated individuals.
For this study, scientists used data from 1,516,702 Omicron and Delta cases that tested positive for COVID-19 between 29th November 2021 and 9th January 2022. 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 several lives lost from Omicron variant epidemics, particularly in populations with high immunity due to vaccination.
Professor Neil Ferguson, Imperial College London, said: “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 hospitalization 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 most significant 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 that Omicron is less severe than Delta and that vaccination substantially reduces severe disease for both variants. But these results should be interpreted because 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. Therefore, getting three vaccine doses is crucial for reducing the burden on healthcare systems.”
- Tommy Nyberg et al. Comparative analysis of the risks of hospitalization and death associated with SARS-CoV-2 omicron (B.1.1.529) and Delta (B.1.617.2) variants in England: a cohort study. DOI: 10.1016/S0140-6736(22)00462-7