Strain XBB.1.5 — a “sub lineage” of the Omicron variant — is not known to carry mutations associated with increased severity, but assessments are continuing, the World Health Organisation said in its latest update on Wednesday.
However, the Kraken and another variant from the Omicron family, known as CH. 1.1, do have a “growth advantage” and could become the dominant variants in the country.
XBB.1.5 is “the most transmissible sub-variant which has been detected yet”, said the WHO’s Covid-19 technical lead, Maria Van Kerkhove.
At present, the dominant variant in the UK is another Omicron sub lineage known as BQ.1.
A new technical briefing from the UK Health Security Agency- (UKHSA) states that XBB.1.5 “remains at very low prevalence in the UK, so estimates of growth are highly uncertain”.
But it adds: “CH. 1.1 and XBB.1.5 are currently the variants most likely to take over from BQ. 1 as the next dominant variant in the UK, unless further novel variants arise. Neither have been designated as variants of concern by UKHSA.”
XBB. 1.5 is estimated to have made up 4.5 per cent of all Covid cases between December 26 and January 1 in the UK.
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“Through our genomic surveillance, we continue to see evolution of variants in the Omicron family,” said Meera Chand, director of clinical and emerging infections at UKHSA.
“UKHSA is constantly monitoring the situation and working to understand the implications for public health.
“Vaccination remains our best defence against future Covid-19 waves, so it is still as important as ever that people come forward and take up all the doses for which they are eligible as soon as possible.”
Susan Hopkins, chief medical adviser at UKHSA, said children should stay at home if they are unwell and have a fever.
“It’s important to minimise the spread of infection in schools and other education and childcare settings as much as possible,” she said.
“If your child is unwell and has a fever, they should stay home from school or nursery until they feel better and the fever has resolved."
What is XBB.1.5?
XBB.1.5 is a descendant of the Omicron XBB sub-variant — which is itself a cross between two previous strains: BA.2.75 and BA.2.10.1.
The original XBB variant has already caused waves of infection in countries such as Singapore and India since the WHO raised concerns in October.
XBB. 1.5, which includes an additional mutation, was initially detected in the US in New York and Connecticut that month.
It has since been found in 38 countries.
Scientists pointed out that the subvariant has a much stronger affinity to ACE2, a key receptor for the virus, which allows it to bind more easily and boosts transmissibility.
XBB.1.5 is attracting attention because it is exhibiting signs of an ability to evade natural immunity or previous protection provided by vaccines, and reinfect people who have recovered from an earlier bout of Covid-19.
Data remains limited on XBB.1.5’s severity and its propensity to cause severe illness or death.
Previous methods to tackle Covid — such as monoclonal antibody treatments — were rendered ineffective by previous strains. That trend is set to continue with the new strain.
According to most recent risk assessment by the WHO, released on Wednesday: "Along with BQ.1 variants, XBB variants are the most antibody-resistant variants to date.
"Using pseudotyped virus neutralisation assays, XBB.1.5 is shown to be as equally immune evasive as XBB.1, the Omicron subvariant with the highest immune escape to date."
The WHO said there was no data on real-world vaccine effectiveness against severe disease or death but assessments were continuing.