UK cases of 'Stealth Omicron' have doubled in the last couple of days as a new study found the new Covid variant is 33% more transmissible.
The sub-variant of Omicron, called BA.2, is spreading fast according to latest UK genetic sequencing which identified 4,294 cases as of last Friday (Jan 28).
Known UK cases stood at 1,431 just four days earlier on Monday January 24 before the UK genomic database recorded 1,993 cases on January 26.
Previous estimates had expected a slower UK doubling rate of every five to six days.
It has been dubbed Stealth Omicron because it cannot be flagged up by standard PCR tests, meaning the real number of infections circulating is likely to be much higher.
A new study from Denmark - which is the epicentre for the new subvariant which has become dominant there - suggests it could soon take over from original Omicron, official name BA.1.
Analysis of infections in 8.500 Danish households between December and January found people infected with BA.2 were 33% more likely to infect others when compared to those infected with BA.1.
Lead study author Frederik Plesner, of Copenhagen University, said: “If you have been exposed to Omicron BA.2 in your household, you have 39% probability of being infected within seven days. If you instead had been exposed to BA.1, the probability is 29%.
“We conclude that Omicron BA.2 is inherently substantially more transmissible than BA.1, and that it also possesses immune-evasive properties that further reduce the protective effect of vaccination against infection.”
The Danish study, which has not yet been peer-reviewed, was conducted by researchers at the universities Statens Serum Institut (SSI).
Denmark and the UK have some of the best national genomic sequencing capabilities in the world and so are likely to be one of the first countries to flag up the threat posed by new variants.
Worldwide the BA.1 subvariant accounts for more than 98% of known Omicron cases, but its close cousin BA.2 quickly became the dominant strain in Denmark, overtaking BA.1 in the second week of January.
BA.2 cases have also been registered in the US, Sweden and Norway, but to a much lesser extent than in Denmark, where it accounts for roughly 82% of cases.
Preliminary analysis by SSI has shown that there is no difference in the risk of hospitalisation for BA.2 compared to BA.1.
Despite the high growth rate of BA.2, overall number of cases are still relatively low.
Covid-19 Genomics UK Consortium data shows during the last 28 days BA.2 made up less than 2% of genetically sequenced Covid test samples.
In an update on Friday the UK Health Security Agency confirmed BA.2 has a “substantial” growth advantage over BA.1.
However the agency said there was evidence that the vaccines hold up slightly better against this new subvariant compared to BA.1.
Its analysis found a Covid-19 booster shot is 63% effective against mild illness with Omicron, but this rises slightly to 70% versus BA.2.
It added there is currently no suggestion that it leads to more severe disease.
UKHSA chief medical adviser Dr Susan Hopkins said: “We now know that BA.2 has an increased growth rate which can be seen in all regions in England. We have also learnt that BA.2 has a slightly higher secondary attack rate than BA.1 in households.
“Although hospitalisations and deaths remain low, cases are still high in some areas and some age groups so it’s important that we continue to act cautiously as restrictions are lifted.”
