Current vaccines should still be effective against severe outcomes of infection with NB.1.8.1 or XFG, although it remains to be seen how those lineages evolve now that they are circulating among hosts on a global scale.

https://www.thelancet.com/journals/laninf/article/PIIS1473-3099(25)00308-1/fulltext

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Officially, these are all "Omicron" variants, but so are thousands of others that have evolved since the last Greek letter was given in November 2021. The reality is that "Omicron" is massively diverse -- far, far, far more so than all the other named variants combined.

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Nimbus (NB.1.8.1) has a more complex ancestry, including three recombination events. Its ancestors include both a chronic infection variant (Pirola) and other recombinants including descendants of XBB (relatives of Kraken).

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Stratus (XFG) is a recombinant of LF.7 x LP.8.1.2 -- both of which descendants of BA.2.86, which itself had evolved within someone infected for ~2 years.

LF.7 = JN.1.16.1.7 = BA.2.86.1.1.16.1.7

LP.8.1.2 = JN.1.11.1.1.1.3.8.1.2 = BA.2.86.1.1.11.1.1.1.3.8.1.2

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While officially "Omicron" (all Greek letters were given between May and November 2021, and none since), Nimbus (NB.1.8.1) is of course very divergent from its early Omicron ancestors.

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The last variant to receive an informal nickname was BA.2.86 "Pirola" nearly two years ago, back in August 2023. Since then, it has been a prolonged "variant soup" phase, with descendants of BA.2.86 arising, gaining prominence, and then falling in frequency.

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