A “highly transmissible” Covid strain has emerged and is already behind one in 25 cases in the UK, surveillance data suggests.
The strain, given the moniker XBB.1.5, has caused alarm in the US over its quick spread and a recent rise in hospitalisations. It is behind four in 10 cases in the country, up from two in 10 one week ago.
XBB is a subvariant of the Omicron BA.2 variant, and XBB.1.5 is a subvariant of XBB. It emerged as a “recombinant lineage between the second generation Omicron variants”, Professor Kei Sato wrote in a study by University of Tokyo, Hokkaido University and Kyoto University.
The Japanese researchers studied XBB’s characteristics in hamsters including transmissibility and immune resistance.
Their results suggested that the subvariant is highly transmissible and has developed resistance to immunity.
Experts have said that the strain is a “wakeup call” and could worsen the NHS crisis, which has seen the health service battered by a “twindemic” of Covid and flu.
However, officials caution that there is no indication the strain causes more severe illness than earlier variants.
Figures from the Sanger Institute, one of the UK's largest Covid surveillance centres, shows four per cent of cases in the week to 17 December were caused by XBB.1.5. It is the first time the strain has been listed on the institute's virus dashboard, which is updated weekly.
Professor Lawrence Young, a virologist at Warwick University, told MailOnline that the emergence of the strain is a “wakeup call” and could exacerbate the NHS crisis.
He said: “The XBB.1.5 variant is highly infectious and is driving increased hospital admissions in New York, particularly among the elderly.
“Waning immunity, more indoor mixing because of the cold weather and lack of other mitigations, such as wearing facemasks, are also contributing to this surge of infection in the US.
“We don't know how this variant is going to behave in the UK in a population that has been previously exposed to other Omicron variants and where many of the over 50s have had booster shots with a bivalent vaccine.
“Nevertheless, this is a wakeup call — a sharp reminder that we can't be complacent about Covid.
“The threat of XBB.1.5 and other Covid variants further exacerbating the current NHS crisis stresses the need for us to remain vigilant.”
The Omicron subvariants have taken the US by storm as together they accounted for 44.1per cent of the total cases in the country for the week ending 31 December.
The subvariants were previously reported as just XBB before this week.
Though the subvariants are currently dominant in the Northeast, they account for fewer than 10 per cent of infections in many other parts of the country, according to the US Centers for Disease Control and Prevention on Friday (CDC).
XBB.1.5 has been detected in at least 74 countries and 43 US states, according to outbreak.info which uses data from the Global Initiative on Sharing Avian Influenza Data (GISAID).
Professor Paul Hunter, an epidemiologist at the University of East Anglia, said that the rapid increase in XBB.1.5’s prevalence “certainly very worrying” and suggests “a pretty dramatic growth advantage and enough to drive a new wave of infections”, he said.
But he noted that the US data is based on estimates and it is difficult to accurately measure Covid data, including variant levels, due to delays in testing and reporting.
Professor Hunter said this makes it “too early to say whether XBB.1.5 will cause big problems” or not, though it will likely become the dominant variant in the UK.
While it has some mutations that allow it to better dodge immunity from previous infections and vaccinations, protection against severe illness, hospitalisation and death have held up 'very well' against similar strains, he said.
“So I doubt XBB.1.5 will cause a major disruption for health services but we need to wait a couple of weeks to see what is happening in the US to be certain,” Professor Hunter added.
