A large UK comparative analysis has found that infection with the Omicron COVID-19 variant leads to less severe outcomes than with delta
Individuals who become infected with the Omicron COVID-19 variant experience less severe outcomes such as hospitalisation and death compared to those with the Delta variant according to the results of a large study by a multi-disciplinary group of UK researchers.
The Omicron COVID-19 variant has produced a surge of infections and been found to be associated with high transmission among household contacts, particularly among those who lived with index patients who were not vaccinated or who did not take measures to reduce the risk of transmission to household contacts. Nevertheless, despite a higher level of transmissibility, studies have also suggested that the Omicron variant is associated with substantial severity of illness in comparison to the Delta variant.
With an increasing number of individuals now fully vaccinated against COVID-19, the purpose of the present study was to offer a more detailed understanding of the overall impact of both less severe outcomes and greater immunity on rates of hospitalisation and mortality during the Omicron wave. The UK team obtained individual-level data on confirmed cases of COVID-19 infection in England between November 2021 and 9 January 2022 and linked these to vaccination status, hospital attendance and admission as well as deaths. During the period of study genomic sequencing was performed for some of the infections which enabled the team to distinguish between the two variants. For the study, a hospital admission was defined as a stay of at least one or more days. The analysis was stratified by age, vaccination status and adjusted for sex, deprivation index and evidence of prior infection.
Omicron caused less severe outcomes
During the period of study there were 4,135,347 confirmed cases of COVID-19, of which 1,526,702 (37%) had information on the infecting variant and consisted of 448,843 Delta and 1,067,859 Omicron cases.
The adjusted hazard ratio (HR) for attendance at hospital, but not necessarily admission, was lower for Omicron than Delta (HR = 0.56, 95% CI 0.54 – 0.58). In addition, compared to Delta, the risk of hospital admission among those infected with Omicron was 59% lower (HR = 0.41, 95% CI 0.39 – 0.43) and the risk of death within 28 days was 69% lower (HR = 0.31, 95% CI 0.26 – 0.37).
When stratified by age, there was no difference between the two variants among those 10 years of age and younger (HR = 1.10, 95% CI 0.85 – 1.42). However, there was a significant difference and reduction among those at least 80 years of age (HR = 0.47, 95% CI 0.40 – 0.56).
For both of the variants, prior infection offered protected against death in those who were vaccinated (HR = 0.47) and unvaccinated (HR = 0.18). Interestingly, among those who were vaccinated, prior infection appeared to offer no additional protection against infection (HR = 0.96, 95% CI 0.88 – 1.04), which was in contrast to those who were unvaccinated but had a previous infection (HR = 0.55, 95% CI 0.48 – 0.63).
The authors concluded that the less severe outcomes observed for Omicron were largely driven by a less severe variant and increased immunity due to vaccination.
Nyberg T et al. 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: a cohort study Lancet 2022