Variant watch: First SA study says Omicron may be less severe but with riders
The study offers the first insights after standardising data based on age and gender, the two factors that dictate severity and hospitalisation risks.
People infected with the Omicron variant in South Africa had 80% lower odds of requiring hospitalisation than those infected with another variant between October and November, a new study released on Wednesday by health experts in the country has found, offering new hope that the new variant of concern (VOC) may be less virulent in some populations.
In a second study and in press briefings later on Wednesday, South African health experts said the apparent reduction in disease severity was likely due to the high rates of prior exposure to Covid-19 in Gauteng, the province where Omicron first became widespread and where the studies were focussed. Both studies are still based on early data and carry a wide confidence range.
The first study found that Omicron-infected people had 80% lower odds of hospital admission when compared to those who caught another variant during the current (fourth) wave of infections, and 70% lower odds of developing severe disease compared to those infected by the Delta variant in the previous (third) wave.
But, importantly, it found that if people infected with Omicron required hospitalisation, their likelihood of developing severe disease was no different to those who were infected with other variants during the ongoing wave.
The study offers the first insights after standardising data based on age and gender, the two factors that dictate severity and hospitalisation risks. It stresses that the findings do not suggest the new VOC is inherently less severe, but may be manifesting in a less intense manner now due to high population immunity from past outbreaks.
The second study found that at least 68% of the unvaccinated population in Gauteng is estimated to have had a past infection prior to the Omicron wave. “The biological basis for the uncoupling of case rates and rates of severe disease with the Omicron-dominant compared with the previous Delta-dominant wave is possibly the extensive cell-mediated immunity in the population induced by previous natural infection and vaccination,” said the second study.
What does this mean?
Cheryl Cohen, professor at the Centre for Respiratory Diseases and Meningitis at the National Institute for Communicable Disease, said during a press briefing that the results likely hold for other regions where a large number of people have been infected, although it is still not clear how Omicron will hit countries where vaccination rates have been high but infection rates were kept low.
“These results are likely generalisable in countries in sub-Saharan Africa which has high levels of prior infection. But what is unclear is whether this picture will be similar in countries where there is high levels of vaccination but very low level of [past] infection – where the baseline epidemiology is very different,” Cohen said, and agreed that population immunity after two years of the pandemic “may be leading to attenuation of severe disease”.
For perspective, India’s seroprevalence among unvaccinated people was estimated at 67% (similar to Gauteng’s) in July following the devastating Delta wave of infections.
Cohen added during the meeting that there were several limitations to the study. First, Omicron infections were estimated on the basis of a test report artefact – Omicron infections do not test positive for the S gene, while other variants do. The so-called S gene target failure (SGTF) happens only in one particular test kit, which meant the findings could be biased towards geographic regions where it was used.
The other limitation was that the analysis was done early in the fourth wave when numbers were small and mild patients were more likely to be admitted.
The South African researchers in the second study identified other possible implications of the trends seen in Gauteng.
First, it appears that cell-mediated immunity (a more advanced line of defence compared to antibodies, which tend wane) may be more important in combating variants.
Second, determining inherent virulence may not be easy at the moment. “The virulence of the Omicron variant relative to other variants remains to be determined, and it is not epidemiologically possible to draw conclusions based on our data due to the differing prevalence of immunity compared with what existed in the past,” they said, implying that most people infected now are either vaccinated or had a past infection.
Anecdotal reports of Omicron infection deaths from other countries suggest that unvaccinated people remain most at risk. This is corroborated by scientific evidence that has found that the virus has not evolved enough to cheat killer T cells (part of cellular immunity) in the same way as it does antibodies (which are part of what is known as humoral immunity),