Covid in SA: People infected with Omicron have an 80% lower risk of hospitalisation, early data shows

  • People infected with Omicron are less likely to end up in the hospital, according to a preliminary analysis by scientists in SA.
  • All current data is pointing to a consistent picture of a reduction in severity with Omicron in South Africa, says one of the study's authors. 
  • It is not yet clear whether this is as a result of Omicron being less virulent, or due to South Africa's high levels of immunity.

Covid-19 data in South Africa is increasingly pointing to the Omicron variant causing less severe illness than earlier variants, researchers at the National Institute for Communicable Diseases (NICD) said on Wednesday.

"Compellingly, together our data suggests a positive story of a reduced severity of Omicron compared to other variants," said Professor Cheryl Cohen, the head of the NICD's Centre for Respiratory Diseases and Meningitis.

Cohen and her colleagues conducted an early assessment of the severity of Omicron infection in the country.

Their findings were posted online to the preprint server medRxiv on Tuesday.

The research team linked databases together and assessed two comparisons: from 1 October to 30 November 2021 - during the emergence of Omicron - they compared the severity of cases between individuals with Omicron infection with those who had non-Omicron infection.

For the second part of their analysis, they compared Omicron infections in this same period to sequencing data of people with Delta infections from April (in the third wave) to November 2021.

Severe disease was defined as a patient requiring one of the following: admission to the intensive care unit (ICU), oxygen treatment, ventilation, extracorporeal membrane oxygenation (ECMO), experienced acute respiratory distress syndrome (ARDS) and/or died.

Results

In the first comparison, the team found people with Omicron infection were significantly less likely to be admitted to hospital, compared to those with Delta infection.

Overall, these individuals had an 80% reduced chance of being admitted to hospital, said Cohen.

However, among patients admitted to hospital, there was no significant difference noted between those who had developed severe disease. In other words, hospitalised patients with Omicron infection had a similar chance of developing severe disease than those infected with other variants.

The second part of the analysis indicated Omicron infections were associated with a 70% lower chance of severe disease compared with Delta infections in the previous wave, said Cohen.

"Both datasets point to less severity when looked at on an individual level," she added. "It's very encouraging data that points strongly to a substantially lower severity in the Omicron wave."

Limitations 

The analysis has some limitations, including a lack of sufficient follow-up data.

Still, Cohen said one of the strengths of their analysis was that they included patients diagnosed from November, with data up to 21 December.

"What that means is that people included were allowed three whole weeks for their outcomes to be assessed … The data seems to be robust and there's no suggestion that this pattern is going to disappear," she added.

The NICD's public health specialist and co-author of the study, Dr Waasila Jassat, said there had been a lower proportion of severe disease in the fourth wave compared to the second and third waves for all age groups.

The rapid increase in cases, therefore, did not translate into a huge increase in hospital admissions, she added.

Does this mean Omicron is less virulent?

While there are signals towards lower severity among those admitted, Jassat said this lower number of severe diseases in the current wave could be due to a number of factors.

"[It could include] the level of prior immunity from people who have already gotten vaccinated or who had natural infection, or it could also be due to the intrinsically lower virulence of Omicron, but we need more studies to be able to unpack these things," she added.   

In their paper, the authors concluded: "Early analyses suggest a reduced risk of hospitalisation among [Omicron]-infected individuals when compared to non-[Omicron]-infected individuals in the same time period, and a reduced risk of severe disease when compared to earlier Delta-infected individuals. Some of this reduction is likely a result of high population immunity."

High levels of natural immunity

It is estimated 60 to 70% of people in South Africa have had a previous infection over the course of the three waves. Currently, around 40% of the population has been vaccinated.

While the researchers could only rely on self-reported vaccination data when conducting their study, Jassat said, on average, most patients admitted to hospital in the fourth wave were unvaccinated.

Omicron behaving in a way that is less severe

"We can't tell from this data how much is [as a result of] vaccination and how much is from previous infection and intrinsic reduced virulence of the virus," added Cohen.

This is because data on vaccination is incomplete and because most previous Covid-19 infections are undiagnosed in the country.

"But in South Africa, this is the epidemiology: Omicron is behaving in a way that is less severe," said Cohen. 

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