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Omicron cases less severe than other waves, hospitalisation rate lower, shows study in Lancet

Pre-print study conducted in Gauteng, South Africa, says infections are likely milder due to a less virulent virus, and high immunity from prior infection(s) or vaccination.

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New Delhi: A pre-print study published in the Lancet journal, observing the clinical severity of Covid-19 patients infected with the Omicron variant, has found that there have been fewer hospitalisations and less severe symptoms compared to previous waves.

The study, published on 29 December, is yet to be peer-reviewed, and has observed cases through the first four weeks of the fourth wave in the province of Gauteng, South Africa, where the variant was first identified.

“Unlike the pattern observed in the Beta and Delta waves, the rise in cases during the Omicron wave was not accompanied by a concomitant rise in hospital admissions,” the study said.

“The percent(age) of cases admitted was 18.9 per cent (7,774/41,046) during the second wave and 13.7 per cent (4,574/33,423) during the third wave, compared to 4.9 per cent (6,510/133,551) during the fourth wave,” it added.

Omicron has been termed a ‘variant of concern‘ by the World Health Organization, which has stated that preliminary studies suggest it poses a higher risk of reinfection and transmission.

“The reasons for the lower admission rates and less severe infections in admitted patients during the Omicron-dominated fourth wave are not known but are likely to be due to a less virulent virus, and high immunity from prior infection(s) or vaccination, especially the large numbers of vaccinated individuals who had prior infection and so have ‘hybrid immunity’,” the study said.

About 30.9 per cent of Gauteng’s population had been vaccinated at the time of the study.

Also read: Has Omicron reached its peak? Perhaps not — it might be the last Covid variant of concern

Severity of disease

The study observed that of the patients hospitalised with the Omicron variant (6,510), the clinical outcomes of 31.8 per cent were not yet known. Of the patients whose outcomes were known, only 28.8 per cent met the criteria for severe disease, compared to 60.1 per cent and 66.8 per cent of patients with the Beta and Delta variants, respectively.

The proportion of patients requiring supplemental oxygen was also lower among patients infected with the Omicron variant, the study found.

“In multivariable analysis, patients admitted in the Omicron-dominated fourth wave were less likely than patients admitted in the Delta-dominated third wave, to have severe disease,” it said.

Vaccination rate

Of the 6,510 people who tested positive with the Omicron variant and were admitted to hospitals, a larger proportion (1,151) are children and adolescents under the age of 20.

“The percent of cases admitted among children and adolescents below 20 years was 7.1 per cent (306/4,304), 3.8 per cent (161/4,217) and 6.1% (1,151/18,817)” in the Beta, Delta, and Omicron waves respectively, the study said.

Only 5.8 per cent of those under the age of 20 had been vaccinated at the time of the study.

“While admission rates during the fourth wave dropped substantially in those aged more than 20 years in whom vaccination coverage was higher, this was not observed in the largely unvaccinated under-20 years age-group,” said the study, adding that this could be attributed to the fact that children have lower rates of prior infection and/or vaccination.

The highest vaccination rates were observed among those aged above 60 (at 58.4 per cent) followed by 40-59-year-olds (54.3 per cent) and 20-39-year-olds (31.7 per cent).

The researchers of the study added that vaccinations “cannot fully account for the markedly lower numbers of severe infections in 20 to 39-year-old individuals, as less than a third of this age group was vaccinated.”

They also stressed that the results so far “may not represent the entire Omicron-dominated wave’s disease severity and may change when patient data for this entire wave are available”.

(Edited by Neha Mahajan)

Also read: How Corbevax and Covovax, the two vaccines newly approved in India, fight Covid


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