The new Arcturus Covid subvariant has arrived in the UK and is thought to be the most infectious strain so far, a study finds.
Arcturus, or XBB.1.16, is nearly 1.2 times as transmissable as XB.1.5, commonly known as Kraken, until now the most infectious subvariant, according to the study by the University of Tokyo.
It is wreaking havoc in India, pushing up overall Covid cases 13-fold in a month, with hospitals now on red alert and compulsory face masks being brought back.
In the UK, it accounts for just 0.4 per cent of cases, according to the GISAID database – but that share is expected to increase considerably.
However, the subvariant, first detected in late January, is not thought to be any more severe than XBB.1.5.
“We show that XBB.1.5 had an effective reproductive number that was 1.17-fold higher than XBB.1.16,” said Kei Sato, of the University of Tokyo.
The reproductive, or “R”, number is a measure of how many an infected person will go on to infect.
He suggested the higher R number could be because the mutations in the new subvariant make it harder for the immune system to mount a defence.
“It’s been in circulation for a few months. We haven’t seen a change in severity in individuals or in populations, but that’s why we have these systems in place,” said Dr Maria Van Kerkhove, of the World Health Organisation.
It is also not thought to any better at evading immunity built up from prior infection or vaccination, although, given the high level of evasiveness Kraken has demonstrated, that may not be of so much comfort, the preprint study suggests.
“Our results suggest that, similar to XBB.1.5, XBB.1.16 is robustly resistant to a variety of [Covid] antibodies…The ability of XBB.1.6 to exhibit profound immune evasion is comparable to XBB.1.5,” Dr Sato said.
The new subvariant has so far been identified at least 27 countries.
It accounts for about 5 per cent of cases in the US and Australia and 12 per cent in Singapore, according to GISAID.
Professor Lawrence Young, a virologist at Warwick University, said: “The [study report] confirms the growth advantage of XBB.1.16 and its ability to partially evade neutralising immunity induced from previous infections with other omicron variants.
“While it’s clear that XBB.1.16 is more infectious and able to spread, omicron-targeted booster shots will provide some protection – stressing the need for those who are eligible (the over 75s and clinically vulnerable) to get their booster shots. And the anti-viral drug paxlovid will continue to work as an effective treatment.”
He added: “The particularly rapid rise of XBB.1.16 in India is a concern given the levels of immunity in that population from previous infections and vaccination. Current reports show no increase in rates of hospitalisations, ICU admissions or deaths but we have to keep an eye of this variant as it continues to spread.
“The worry is that with reduced levels of surveillance in the UK, waning immunity from previous infections and vaccinations, and the general level of complacency, we are not well-equipped to handle another wave of infection.”
Steve Griffin, a virologist at Leeds University, said: “As with all of these new sub-variants, there seems a clear advantage in terms of antibody evasion and infectivity compared with previous strains, which means that we’ll likely see yet another wave. As in 2022 and earlier this year, the size of these waves is difficult to predict.”