Does the Omicron variant lead to milder Covid-19?
It seems so, according to what may be the first large-scale analysis of cases in South Africa, where the newest variant of concern was first seen and is believed to have become widespread.
Discovery South Africa, a private insurer, said the findings were preliminary and based on the first three weeks of the Omicron outbreak in the country where it now accounts for 90% of the cases, and the picture may yet change.
But for the initial period involving over 200,000 cases, it was found the risk of hospitalisation among adults was 29% less compared with the first wave in mid-2020, when the D614G variant (the closest to the ancestor that spread out of Wuhan) caused a Covid-19 epidemic in the country.
“Epidemiological tracking shows a steep trajectory of new infections, indicating Omicron’s rapid spread, but so far with a flatter trajectory of hospital admissions, possibly indicating lower severity,” the company quoted Ryan Noach, its chief executive, as saying. “This lesser severity could, however, be confounded by the high seroprevalence levels of Sars-Cov-2 antibodies in the general South African population, especially following an extensive Delta wave of infections.”
In Gauteng province, where the variant first spread and quickly became widespread, at least 70% of the population is estimated to have had a previous coronavirus infection, the report noted.
The analysis also mirrored a sharp drop in vaccine efficacy in preventing symptomatic disease reported by real-world studies in the UK. Two doses of the Pfizer-BioNTech vaccine are now only 33% effective in preventing symptomatic Covid-19, down from 80% in the case of Delta, the Discovery SA study found.
Crucially, in what is the first vaccine efficacy estimate for risk of hospitalisation, the analysis found a regular Pfizer vaccine regimen offered only 70% protection against severe complications in Omicron cases, significantly lower than the 93% protection if the infection was with the Delta variant.
The Discovery South Africa analysis did not study the effect of booster doses. A study by the UK Health Security Agency last week showed a third dose of an mRNA vaccine lifted vaccine protection against symptomatic disease from 33% to 71-75%, which means a similar or better boost in protection against severe disease is likely with a booster.
“It is important to note that these insights relate to data from the first three weeks of the Omicron-driven wave in South Africa. Therefore, the insights should be considered preliminary, since they may change as the wave progresses,” the insurer said.
The report also flagged a 20% increased risk among children to be hospitalised due to Omicron, although this too could be skewed since most of the cases detected in all age groups were incidental; these people were already in the hospital for other diseases or procedures and were tested for Covid-19.
The analysis also appeared to corroborate an increased risk of reinfection, finding a 40% relative risk with Omicron in people who were in the past infected with the Delta variant, and 60% chance of repeat infection in people who previously caught the Beta variant.