By Alistair Smout
LONDON (Reuters) -A rise in the prevalence of COVID-19 infections in England in the latest week was driven by the dominant Delta variant rather than Omicron, Britain's Office for National Statistics (ONS) said on Friday.
There have been 134 confirmed cases of the Omicron variant in Britain, which has mutations consistent with reduced vaccine efficacy.
Prime Minister Boris Johnson has introduced travel restrictions to slow the spread of Omicron until it is better understood.
The prevalence of COVID-19 infections in England rose to around 1 in 60 people in the week ending Nov. 27 from 1 in 65 reported the previous week, the ONS said, adding that 99% of all sequenced cases were Delta.
"To date, we have not identified any infections compatible with the new Omicron variant (B.1.1.529) among our survey participants," the ONS said.
However, the UK Health Security Agency (UKHSA) said that in the week since, there had been a rise in the proportion of tests displaying a distinctive mutation that distinguishes Omicron from Delta but can appear in other variants.
"This is still a very small number of cases but is being investigated carefully to understand whether it is related to travel, any other variant or whether there is evidence of spread of Omicron beginning in the community," UKHSA said.
The World Health Organization's chief scientist, Soumya Swaminathan, told the Reuters Next conference that Omicron might displace Delta as the dominant variant globally but that it was impossible to predict.
But it is the spread of the dominant and highly transmissible Delta variant, especially among children, that is keeping infection levels high for now.
On Thursday, Britain reported its highest daily infection number since July, and the ONS said infections were highest in children.
Ravi Gupta, Professor of Clinical Microbiology, University of Cambridge, said that it was not a given that Omicron would entirely displace Delta, which might continue to circulate in unvaccinated populations like young children.
"More Delta circulating in kids and then more Omicron circulating proportionately in vaccinated individuals is one scenario that could play out... it will be hard to displace Delta completely," he told Reuters.
"Omicron is not spreading rapidly at the moment, by the look of it... there is probably sustained community transmission, but it's probably just starting."
(Reporting by Alistair Smout; editing by Michael Holden, James Davey and Sandra Maler)