A lower share of people infected with the Omicron coronavirus variant are likely to require hospital treatment compared with cases of the Delta strain, according to healthcare data from South Africa, Denmark, and the UK.
The findings by separate research teams raise hopes that there will be fewer cases of severe disease than those caused by other strains of the virus, but the researchers cautioned that Omicron’s high degree of infectiousness could still strain health services.
The reduction in severe illness was likely to stem from Omicron’s greater propensity, compared with other variants, to infect people who have been vaccinated or previously infected, experts stressed, though the UK studies also hinted at a possible drop in intrinsic severity.
Unvaccinated groups remained the most at-risk but as the vast majority of breakthrough infections and reinfections caused by Omicron are mild, the proportion of all cases that developed severe disease is lower than with other variants. The strain now accounts for a majority of Covid-19 cases in several countries, including the US.
An analysis of English data carried out by researchers at Imperial College found that Omicron was 11 percent less likely to produce severe disease in any given individual after adjusting for factors including age, sex, underlying health conditions, vaccination status and prior infection.
At the population level this translated into a 25 percent reduction in the risk of hospitalization relative to Delta, with the steeper decline due to the fact that Omicron cases are more likely than Delta cases to be among people who have been previously infected or vaccinated, conferring substantial protection against severe disease.
The South African study, carried out by the country’s National Institute For Communicable Diseases, found that among people who tested positive during October and November, suspected Omicron cases were 80 per cent less likely than Delta cases to be admitted to hospital, after adjusting for various factors including previous infection. But researchers stressed they did not account for vaccination status in this analysis, and data on prior infections were unreliable.
A second analysis from the same research team, this time controlling for vaccination status, found that once admitted to hospital, Omicron and Delta cases from recent weeks both had the same likelihood of progressing to a serious condition. The analyses included more than 10,000 Omicron cases and more than 200 hospital admissions.
“There is something going on… in terms of the difference in the immunological response for Omicron vs Delta,” said Prof. Cheryl Cohen, an epidemiologist at the University of Witwatersrand and one of the study’s authors.
She said the findings suggested that breakthrough infections and reinfections from Omicron were “less severe” and that immune protection from T-cells and B-cells “mediated” Omicron’s “progression to severe disease” despite the fall in antibody protection.