London, Dec 18 Amid various studies claiming that Omicron causes only mild disease, a new study shows that there is "no evidence" that Omicron is less severe than the Delta variant of Covid-19.
The new study led by the UK's Imperial College London showed 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," said the researchers.
In the yet-to-be peer-reviewed study, researchers estimated the growth and immune escape of the Omicron variant in England by using data from all PCR-confirmed SARS-CoV-2 cases in England between November 29 and December 11.
Overall, 196,463 people without S gene target failure (likely to be infected with another variant) and 11,329 cases with it (likely to be infected with Omicron) were included in the SGTF analysis, as well as 122,063 Delta and 1,846 Omicron cases in the genotype analysis.
The report looked at factors associated with testing positive for Omicron compared to non-Omicron (mostly Delta) cases.
The results suggest that the proportion of Omicron among all Covid cases was doubling every two days up to December 11, estimated from both S-gene Target Failure and genotype data.
Based on these results the researchers estimated that the reproduction number (R) of Omicron was above 3 over the period studied.
Further, in the pre-Omicron era, the UK study of Covid infection in healthcare workers estimated that prior infection afforded 85 per cent protection against a second Covid infection over six months.
The reinfection risk estimated in the current study suggests this protection hasA fallen to 19 per cent against an Omicron infection.
"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 researchers said, adding however, hospitalisation data remains very limited at this time.
The latest modelling also pointed out that the Omicron variant largely evades immunity from past infection or two vaccine doses.
"This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity given by both infection or vaccination. This level of immune evasion means that Omicron poses a major, imminent threat to public health," said Prof Neil Ferguson from Imperial College London.
The team 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).
However, the vaccine effectiveness estimates against symptomatic Omicron infection after booster dose was between 55 per cent and 80 per cent, the researchers said.
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