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Omicron infection causes fewer hospitalisations and severe disease compared with Delta variant

Rod Tucker
24 December, 2021  

Omicron infection appears to be associated with a lower risk of hospitalisation and less severe disease than the Delta COVID-19 variant, according to a recent study

According to a preprint study by researchers from the Centre for Respiratory Diseases and Meningitis, Johannesburg, South Africa, Omicron infections appear to be associated with reduced risk of hospitalisation and after admission, less severe disease than the Delta variant.

While the Omicron COVID-19 variant was only identified in November 2021, there has been a flurry of research activity directed at trying to understand its transmissibility, the level of disease severity and effect on healthcare services. While initial laboratory studies have indicated that the variant can escape neutralisation by antibodies generated in fully vaccinated and boosted sera samples, these studies cannot foretell the clinical impact of the variant.

The South African researchers undertook a data linkage study of COVID-19 infections, case data and genomic information to establish whether an Omicron infection was associated with higher rates of hospitalisation and more severe disease among those who were hospitalised in comparison to the Delta COVID-19 variant. Although whole genome sequencing was not used, they utilised the presence of a S Gene Target Failure (SGTF), which serves as a proxy for Omicron and samples were examined between 1st October 2021 and 6th December 2021. Positive infections were therefore classified as either an SGTF or non-SGTF. The severity of an Omicron infection was assessed by comparison of SGTF and non-SGTF infections and to infections known to be caused by the Delta variant and regression models created and adjusted for several factors known to be associated with hospitalisation (e.g., age, sex, co-morbidities).

Findings

During the period of study there were 161,328 COVID-19 cases recorded and the proportion of SGTFs increased from 3% (early October 2021) to 98% (early December).

A total of 11,495 hospital admissions occurred with 2.5% due to an SGTF compared to 12.8% with a non-SGTF (p < 0.001). Multivariate analysis revealed that individuals with a SGTF had a lower odds of hospitalisation (adjusted odds ratio, aOR = 0.2, 95% CI 0.1 – 0.3, p < 0.001). In addition, once hospitalised, the odds of having severe disease were also reduced in those with SGTF although there was uncertainty over this estimate given the wide confidence intervals (aOR = 0.70, 95% CI 0.30 – 1.40).

Finally, the researchers found that compared to infection with the Delta variant, those with an SGTF had a significantly lower risk of severe disease (aOR = 0.30, 95% CI 0.20 – 0.50, p < 0.001).

The authors suggested that these data suggested that an Omicron infection was probably less severe than other variants such as Delta but recognised that this reduced severity might be accounted for by the higher levels of population immunity due to either natural infection and/or vaccination.

These results are broadly similar to those of a second recently published preprint from Scotland which found a nearly 70% reduced risk of hospitalisation (expected ratio = 0.32, 95% CI 0.19 – 0.52) for those with an Omicron infection. The study also found that giving a third or booster dose was associated with a 57% reduced risk of symptomatic infection.

Although both studies are preliminary, they do suggest the possibility that infection with the new variant is potentially less severe than other forms.

Citation

Wolter N et al. Early assessment of the clinical severity of the SARS-CoV-2 Omicron variant in South Africa. MedRxiv 2021