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There is "no evidence" that Omicron has a lower severity than the Delta strain, according to a study in the United Kingdom which also found that the new variant of coronavirus largely evades immunity from past infection or two vaccine doses.
The study by researchers from Imperial College London in the UK estimates that the risk of reinfection with the Omicron variant is 5.4 times greater than that of the Delta variant.
This implies that the protection against reinfection by Omicron afforded by past infection may be as low as 19 per cent, the researchers said.
"The study finds no evidence of Omicron having lower severity than Delta, judged by either the proportion of people testing positive who report symptoms, or by the proportion of cases seeking hospital care after infection," the authors of the study said.
"However, hospitalisation data remains very limited at this time," they added.
The yet-to-be-published study used data for all PCR-confirmed SARS-CoV-2 cases in England who had taken a COVID-19 test between November 29 and December 11, 2021.
The research included people identified as having Omicron infection due to an S gene target failure, as well as people with genotype data that confirmed Omicron infection.
Overall, 196,463 people without S gene target failure -- those likely to be infected with another variant -- and 11,329 cases with it were included in the analysis, as well as 122,063 Delta and 1,846 Omicron cases in the genotype analysis.
The results suggest that the proportion of Omicron among all COVID cases was doubling every two days up to December 11, the researchers said.
They estimate that the reproduction number (R) -- the number of cases directly caused by an infected individual -- of Omicron was above 3 over the period studied.
"This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination," said Neil Ferguson, a professor at Imperial College London.
"This level of immune evasion means that Omicron poses a major, imminent threat to public health," Ferguson said in a statement.
Controlling for vaccine status, age, sex, ethnicity, asymptomatic status, region and specimen date, Omicron was associated with a 5.40 fold higher risk of reinfection compared with Delta, the study found.
The reinfection risk estimated in the current study suggests this protection has fallen to 19 per cent against an Omicron infection, according to the researchers.
They found a significantly increased risk of developing a symptomatic Omicron case compared to Delta for those who were two or more weeks past their second vaccine dose, and two or more weeks past their booster dose for AstraZeneca and Pfizer vaccines.
"Depending on the estimates used for vaccine effectiveness against symptomatic infection from the Delta variant, this translates into vaccine effectiveness estimates against symptomatic Omicron infection of between 0 per cent and 20 per cent after two doses, and between 55 per cent and 80 per cent after a booster dose," the researchers added.