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expert reaction to preprint looking at an early assessment of the clinical severity of the Omicron variant in South Africa

A preprint, an unpublished non-peer reviewed study, from South Africa provides an early assessment of the clinical severity of the SARS-CoV-2 Omicron variant in South Africa.


Prof Paul Hunter, Professor in Medicine, UEA, said:

“To my knowledge this is the first properly conducted study to appear in preprint form considering whether or not omicron is more or less likely to cause severe disease than delta.  There were two main outcomes – risk of hospital admission, and also probability of severe disease after being admitted to hospital.  The authors adjusted for various confounding factors that could influence the results such as age, sex and whether it was a known reinfection.  And for severity of disease after admission they also adjusted for the presence of other illnesses, “comorbidities”, and prior immunisation amongst others.

“The authors found that the adjusted odds ratio for admission was 0.2, roughly equivalent to about an 80% reduction in risk of hospital admission in omicron compared to delta.  They report that the adjusted odds ratio for severe disease outcome in people after hospitalisation was 0.7, roughly equal to about a 30% reduction in risk of severe disease in patients in hospital.  But the confidence intervals were wide and so we cannot say that severity was indeed less in hospital patients with omicron.

“This is an important study but its main weakness is that uses “historic controls” .  All omicron cases were identified between 1 October and 30 November 2021 and Delta infections diagnosed between April and November 2021.  Studies using such historic controls are generally consider weak as the results could be biased by time issues.  So even though cases of omicron were less likely to end up in hospital than cases of delta, it is not possible to say whether this is due to inherent differences in virulence or whether this is due to higher population immunity in November compared to earlier in the year.  To a certain extent this does not matter to the patient who only cares that they won’t get very sick.  But is important to know to enable improved understanding of the likely pressures on health services.”



Preprint title: ‘Early assessment of the clinical severity of the SARS-CoV-2 Omicron variant in South Africa’ by Nicole Wolter et al. This work is not peer-reviewed.



Declared interests

None received.

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