Covid XBB.1.5 (the Kraken Wakes*)

A new and highly transmissible subvariant of the Omicron Covid18 variant, XBB.1.5, also nicknamed the Kraken, was first identified in the USA in late October 2022. At the beginning of December, Omicron sub variant XBB.1.5 made up just 1.3 per cent of all COVID-19 cases in the U.S. By the week ending 10 February 2023, XBB.1.5 made up an estimated 75% of cases in the USA. Although XBB.1.5 is around 12% more transmissible than over forms of Omicron, there is no evidence that it is any more severe than other forms of Omicron.

Analysis from the COVID-19 Genomics UK Consortium also suggests that XBB.1.5 now makes up 31 per cent of COVID cases in the UK ( as of 9th February) – that’s double the week before. The European Centre for Disease Prevention and Control (ECDC) has predicted that there is a moderate probability that XBB.1.5 will account for over 50% of infections in Europe within the next month or two. Few cases have been detected to date in Australia, and its unclear whether it will outcompete other variants as it has done in the USA.

As of 15 February 2023, there were 1,309 named variants and subvariants of Covid-19 listed in the PANGO database and some scientists have started to use nicknames from mythology to make it easier to talk about variants of public interest. XBB.1.1.5 has been given the nickname Kraken, a sea-monster from Norse mythology. Other Omicron subvariants deemed worthy of a name include BA.2.20 (the Basilisk), XBB.1 (the Hippogryph) and BF.7 (the Minotaur). Other Covid experts have objected to the use of these nicknames, and the Kraken in particular, saying it will cause unnecessary fear in the general public.

The Kraken (XBB.1.5) descends from XBB (the Gryphon), which emerged in the spring or summer last year, possibly in India. The Gryphon was the product of two different forms of Omicron that both infected someone. As they were replicating inside that person, their genes were mixed together, and then we got a new hybrid. And this hybrid is very good at evading defences from vaccines and infections.

So it caused a big surge in Singapore in the fall, but it didn’t really become that common elsewhere because it was competing with so many other subvariants. But as it multiplied, it started gaining more mutations. So XBB gave rise to XBB.1, and then XBB.1 mutated again into XBB.1.5. And it looks like XBB.1.5 gained a really crucial mutation in the spike protein, which makes it more transmissible on top of doing a better job of escaping antibodies.

Preliminary studies indicate that the Kraken has very limited susceptibility to vaccine antibodies from the original mRNA vaccines and boosters. The protection given by bivalent boosters (second generation vaccines, which became available around October last year] is better. Preliminary studies suggested protection was reduced relative to other Omicron variants – some estimating the efficacy was only 30 per cent or less. However, the first study to specifically compare bivalent protection against the Kraken with protection against other Omicron variants found that it almost equally protective.

Compared to those who received 2-3 monovalent doses only, the bivalent vaccine effectiveness was 43% against Omicron and 40% effective against Kraken in 50-64 year olds – this difference was not statistically significant. A vaccine effectiveness of 40% means that the vaccine reduces the risk of infection by 40% (in this case compared to those with monovalent vaccinations). The bivalent effectiveness in those over 65 was similar to those 50-64 and not statistically different between Kraken and other Omicron. The bivalent effectiveness was even higher in those under 50, at around 50%.

Findings from this study suggest that bivalent booster doses are continuing to provide additional protection against symptomatic infection by Kraken similar to that for other Omicron, for at least the first 3 months, after vaccination in persons who had previously received 2, 3, or 4 monovalent vaccine doses. Its definitely worth getting the bivalent booster!

However, while XBB.1.5 has reduced vaccine protection against infection (and developing symptoms and long COVID), vaccines “still remain effective against severe disease”, according to the ECDC.


* The Kraken Wakes is the title of an apocalyptic science fiction novel by John Wyndham, first published in the UK in 1953. It describes the invasion of earth by aliens who can only survive in the deepest parts of the ocean. Wyndham was one of my favourite SF authors when I was a teenager and is best known for another apocalyptic SF novel, the Day of the Triffids, in which humanity is almost wiped out by a carnivorous plant species.

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