Less severe, therefore, but more transmissible. As early as December 2021, early data from South Africa suggested that the Omicron variant was less virulent than its predecessor, the Delta variant of the SARS-CoV-2 virus. Since then, studies have continued to confirm this reduced severity. Because it is spreading like wildfire, Omicron now largely dominates the chart of new infections globally.
A very large study, published on February 4 in preprint on the journal’s website The Lancet, specifies the extent of this drop in severity. A total of 1,516,533 patients affected by Covid-19 were included in this analysis, conducted in the UK between November 22, 2021 and January 9, 2022. Of these, 1,067,736 were infected with Omicron and 448,797 with Delta. Coordinated by teams from Imperial College London and the University of Cambridge, the study compared hospitalization rates (within fourteen days of infection) and death rates (within twenty-eight days after infection) related to Omicron or Delta. The study took into account the vaccination status, age, ethnic origin, sex, the existence or not of a previous infection, the region and an index measuring the socio-economic level of the patients.
By its statistical power and its methodology, “This is the most comprehensive study on the subject, notes Mahmoud Zureik, professor of epidemiology and public health at the University of Versailles-Saint-Quentin-en-Yvelines. It concerned 37% of the positive cases identified over this period across the Channel. Another asset, the Omicron and Delta cases were compared over the same period”. During this period, 9,623 patients infected with Omicron were hospitalized, and 7,356 patients infected with Delta.
First result: the risk of hospitalization linked to Omicron is 2.4 times lower compared to Delta. It is among the 50-69 year olds that it falls the most: it is divided by 4 (among the over 80s, it is divided by 2.1). As for the risk of death, all ages combined, it is 3.2 times lower. According to age, it is divided by 3.6 among 30-39 year olds; by 4 among 40-49 year olds; by 6.25 among 50-59 year olds; by 4.6 among 60-69 year olds; by 3.9 among the 70-79 year olds and by 2.2 among the over 80s.
The booster dose, less effective than against Delta
This lesser severity of Omicron, however, does not benefit the younger ones. “In children under 10, the severity of Omicron-related infections did not differ from that of Delta-related infections,” notes Mr. Zureik. Among 10-19 year olds, the risk of Omicron-related hospitalization decreases by 17%, a non-significant difference. Over the period studied, the authors identified 475 hospitalizations related to Omicron in children under 10 years old, and 456 in 10-19 year olds – but “only” two and one death in these two age groups.
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