Covid-19: Early SA data suggests severe disease among Omicron patients 73% less likely than Delta

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  • Less people were admitted to hospital in Gauteng's Omicron wave of Covid and cases were less severe, according to research.
  • Gauteng's first four weeks of Omicron cases were analysed against the same periods in the Delta and Beta waves of Covid.
  • The authors cautioned that the trends should not be extrapolated to other countries with different population profiles.

The number of Covid-19 people admitted to hospital in Gauteng's Omicron-driven fourth wave were lower and their cases were less severe than those admitted during the province's third wave of Delta-driven infections, early SA data shows.

Researchers analysed trends from the first four weeks of cases in the fourth wave in Gauteng – the epicentre of the Omicron outbreak. They compared this to data for the same periods during the second wave, driven by the Beta variant, and the third wave, driven by the Delta variant.

The study involved scientists at the National Institute of Communicable Diseases (NICD).

Writing in The Lancet preprint server, the researchers noted:

Admitted patients in the Omicron-dominated fourth wave were 73% less likely to have severe disease than patients admitted during the Delta-dominated third wave.

However, they cautioned that the findings should not be extrapolated to other countries with different population profiles, and prior infection and vaccination coverages.

They explained:

"Since any combination of a less-virulent virus, comorbidities, high immunity from prior infection(s) or vaccination may be important contributors to this clinical presentation, care should be taken in extrapolating this to other populations with different co-morbidity profiles, prevalence of prior infection and vaccination coverage." 

Gauteng reached the peak of its fourth wave last week, health officials and the NICD said. In a statement on Thursday, the government said indicators suggested that the country might have passed the peak of the fourth wave at a national level – but cases declined in all provinces except the Western Cape and Eastern Cape, it said. 

Detailed findings

The current analysis included the following periods:

  • Second (Beta) wave: 29 November – 26 December 2020
  • Third (Delta) wave: 2 May – 29 May 2021
  • Fourth (Omicron) wave: 14 November – 11 December 2021

While the second wave saw 41 046 reported cases during the analysed period, the third wave had 33 423 cases. The fourth wave, driven by the highly transmissible Omicron variant, saw a massive increase of 133 551 infections.

However, of these cases, 7 774 (18.9%) people were admitted to hospital during the second wave; 4 574 (13.7%) during the third wave; and 6 510 (4.9%) during the fourth wave, respectively.  

Shedding further light on this, the researchers broke down the numbers of those who had severe disease – defined as needing acute respiratory distress, supplemental oxygen, mechanical ventilation, high/intensive care or death:

  • Beta wave: 60.1%
  • Delta wave: 66.9%
  • Omicron wave: 28.8%

It's worth noting that out of the 6 510 Covid-19 patients who required admission in the fourth wave, 2 072 were still in hospital and did not yet have a documented in-hospital outcome at the time the study was submitted.

The median hospital stay was also lower in the Omicron wave (four days), than the Delta (eight days) and Beta (seven days) waves.

Findings supported

The results support the epidemiological data experts have been observing during the current wave. On 22 December, NICD scientists presented the findings of their preliminary study posted online. 

"It's very encouraging data that points strongly to a substantially lower severity in the Omicron wave," said professor Cheryl Cohen, the head of the NICD's Centre for Respiratory Diseases and Meningitis.

The researchers echoed the statements other SA researchers made in the last few weeks. They wrote: "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 (disease severity) 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.'"

It is estimated that more than 70% of people in SA have had Covid-19 since the beginning of the epidemic. Around 45% of the population is vaccinated.

Another potential explanation

The researchers also pointed to a tissue-based study which showed that Omicron infects the cells of the bronchus faster but cells of the lung slower than Delta, which may explain the less severe infections in the fourth wave.

Although early surveillance data paints a promising picture of low disease severity with Omicron, they encouraged further research looking into the impact of these various contributors.

"The role of prior immunity from natural infection, vaccination and/or lower virulence needs to be investigated as all these factors may be contributing to some extent," they wrote.

This week, researchers at the University of Cape Town posted a study that showed that most of the disease-fighting T cells acquired through vaccination and prior infection still recognise Omicron and should, therefore, offer protection against serious illness, Health24 reported. A US study, posted a few hours later, had similar results.

*For more Covid-19 research, science and news, click here. You can also sign up for our Daily Dose newsletter here.

READ | High immunity in SA leading to fewer hospitalisations during Omicron wave, analysis suggests

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