Scientists involved with the study stressed that “hospitalisation data remains very limited at this time” and said it “may take several weeks to fully understand” the fast-spreading variant’s severity.
There is currently no evidence that the Omicron variant of coronavirus causes less severe disease than Delta, according to an early study of UK infection data – which also found a fivefold risk of reinfection and highlighted the need for booster jabs.
Studying NHS and Health Security Agency (UKHSA) records of those with a positive PCR result in England between 29 November and 11 December, Imperial College London scientists found that there was not yet any significant change in the proportion of people developing symptoms or requiring hospitalisation with the Omicron variant compared to the Delta.
While Imperial’s findings – which were backed up by separate UKHSA analysis also published on Friday – run in contrast to early reports from South Africa suggesting Omicron could cause milder illness, they echoed previous estimates of the variant’s impact on vaccine efficacy.
Imperial scientists estimated that two vaccine doses could offer 0 to 20 per cent protection against symptomatic Omicron infection – with protection of between 55 and 80 per cent after a booster dose. This data was dependent upon estimates of vaccine effectiveness against Delta.
“While we are yet to fully understand the implications of Omicron in terms of disease severity, boosters will be a key part of the policy response for reducing hospitalisations and deaths, and to prevent health systems from becoming overburdened,” said Dr Alexandra Hogan, of Imperial.
The new report from Imperial’s World Health Organisation collaborating centre, which is not yet peer-reviewed, also found the risk of reinfection with Omicron is 5.4 times greater than with Delta – implying the protection that past infection offers against reinfection by Omicron may be as low as 19 per cent.
“This study provides further evidence of the very substantial extent to which Omicron can evade prior immunity, given by both infection or vaccination,” said Professor Neil Ferguson, also of Imperial.
“This level of immune evasion means that omicron poses a major, imminent threat to public health.”
The distribution of Omicron by age, region and ethnicity also currently differs from Delta, according to the findings – with 18-29-year-olds, residents in London, and those of African ethnicity having significantly higher rates of infection with the new variant compared with its predecessor.
This “very different” pattern of Omicron clusters in younger adults, “who are inherently less likely to be hospitalised or to die”, limits the extent to which the severity of two variants can be compared, said Professor Penny Ward, a visiting professor in pharmaceutical medicine at King’s College London who was not involved with the study.
Imperial’s findings on potential severity were lent further weight by the UKHSA, who on Friday issued a fresh report on its ongoing research and surveillance of the variant.
While stating that data to fully assess Omicron’s severity remains insufficient, the health agency updated its risk assessment of the new variant to add: “There is no signal that supports a difference in the intrinsic virulence of the omicron virus compare to Delta.”
The government agency estimated that 7.6 per cent of Omicron cases are reinfections, compared with 1.5 per cent of other variants – which would mark a roughly threefold rise, said Professor Meaghan Kall, a lead epidemiologist at the UKHSA.
This was slightly lower than Imperial’s estimate that 11 per cent of omicron cases are reinfections, and the agency said that “overall reinfection rates remain low”.
Studying the growth rate of the new variant, the government scientists also estimated that Omicron’s doubling time is now less than two days – and could be close to 1.5 days across parts of the north of England.
As the variant fast becomes the dominant variant across in parts of the UK – with Scotland becoming the first to declare this on Friday afternoon – and infections soared to new all-time highs for three straight days, the sheer scale of those testing themselves for the virus has prompted some to raise questions over whether lateral flow tests are as capable of detecting Omicron effectively.
But the UKHSA has previously said there was no data to support this theory, and its review on Friday said that laboratory evaluation of five lateral flow brands likely to be deployed by Test and Trace, including Innova, SureScreen and FlowFlex, found they were all able to detect Omicron infection.
