BA.2 Omicron variant no more severe than predecessor, study shows
Those infected with ‘stealth Omicron’ are not more likely to be hospitalised, researchers find
The Omicron BA.2 sub-variant of SARS-CoV-2 is no more virulent than the BA.1 lineage it has displaced in SA, according to new research from scientists at the National Institute of Communicable Diseases (NICD).
“Very reassuringly, BA.2 has the same feature as BA.1 — the risk of hospitalisation is less than previous variants. And among patients admitted to hospital, BA.2 is behaving very much like BA.1,” said Cheryl Cohen, head of the NICD’s centre for respiratory diseases and meningitis and co-author of a paper describing the research that will be published on the preprint server MedRxiv later this week. Preprint servers publish research before it has been peer reviewed.
The first Omicron variant detected by scientists, dubbed B.1.1.529 or BA.1, was until recently considered the most transmissible lineage and was responsible for driving SA’s fourth wave of infections late last year. But its close relative, BA.2, nicknamed “stealth Omicron”, has proven even more infectious and has quickly gained ground in many parts of the world, prompting fears it might cause more severe disease too.
The BA.1 sub-variant of Omicron was first detected in SA in November. By mid-February it had been largely replaced by BA.2, which accounted for 86% of the SARS-CoV-2 genomes sequenced in SA, according to Cohen.
However, the growing dominance of BA.2 has not triggered a significant rise in new cases in SA, and it has now been found in a large study of Covid-19 patients those infected with BA.2 are no more likely to be hospitalised than those with BA.1. Equally reassuring, hospitalised patients with BA.2 have the same odds of becoming severely ill as those with BA.1, said Cohen.
The researchers exploited the fact one of the most widely used PCR tests in SA does not detect one of the three target genes in BA.1 (a phenomenon known as S-gene target failure) but does detect all three target genes in BA.2. They used this feature as a proxy to distinguish between patients infected with the two sub-variants.
Among 95,470 confirmed cases that had been tested with the TaqPath™ Covid-19 PCR assay, 3.6% of those with an S-gene positive infection (the proxy for BA.2) were hospitalised compared with 3.4% with an S-gene target failure infection (the proxy for BA.1).
After controlling for other factors associated with hospitalisation, such as age and comorbidities, the researchers found the risk of being admitted to hospital was the same for people infected with BA.1 and those infected with BA.2.
And among patients hospitalised between December 1 2021 and January 20, the odds of developing severe disease did not differ between the two sub-variants, said Cohen.
She cautioned against extrapolating the findings to other settings, as SA has a very high level of immunity from natural infection compared with many other parts of the world, where a greater proportion of the population had vaccine-induced immunity.
More than 70% of blood donors have antibodies produced in response to natural infection, according to data released on Wednesday by the SA National Blood Service.
Its analysis of blood donated in eight provinces (it excluded the Western Cape) showed the prevalence of SARS-CoV-2 antibodies produced by natural infection ranged from 63.4% in the Northern Cape to 80.7% in the North West.
By Tuesday evening, barely 28% of SA’s population had been fully vaccinated, according to government figures. Among adults, the figure stood at 41.8%.