Thread by @TRyanGregory

Meet "Nimbus", aka SARS-CoV-2 variant NB.1.8.1.

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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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A prolonged variant soup phase involving the Pirola clan does not mean there was no within-host evolution occurring. It just meant that nothing had gotten back into the general population that could compete with the many, many descendants of BA.2.86.

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Nimbus (NB.1.8.1) gets its name from the type of cloud (plus it's helpful that it contains "n" and "b"). It is not just another member of Pirola clan. Nimbus is a triple recombinant that includes both Pirola (BA.2.86) and Kraken (XBB) variant ancestry.

Nimbus has been significant in parts of Asia for some time, and has since begun spreading around the world. It was labeled as a "variant under monitoring" (VUM) by WHO as of May 23, 2025.

forbes.com/sites/brucelee…

Here is a map showing the spread of Nimbus (NB.1.8.1) courtesy of variant tracker @Mike_Honey_.

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.

In terms of current vaccines, Moderna and Pfizer target KP.2 (= JN.1.11.1.2) and Novavax targets JN.1. Recent recommendations were to keep them the same, though it looks lime they may be going with LP.8.1, which is not particularly close to NB.1.8.1.



8/nbcnews.com/health/health-…

Oh, and yes we're aware of multiple pop culture references to "Nimbus", including Harry Potter, Futurama, Rick and Morty, etc.

Here's a preprint with some more characteristics of Nimbus NB.1.8.1.

biorxiv.org/content/10.110…

Quick clarification, Kraken was specifically XBB.1.5, but the ancestors of Nimbus were XBB.1.9 and XBB.1.19. I should have worded it slightly differently as "close relatives of Kraken (i.e., descendants of XBB)".

One other clarification. JN.1, KP.2, LP.8.1, and NB.1.8.1 share key mutations so it's not that those vaccines wouldn't work against NB.1.8.1 as it is now. My point was that Nimbus is a new lineage, not just part of the same Pirola soup, and it remains to be seen how it evolves.

@pelagicbird I'll be getting a booster in any case. However I don't know that LP.8.1 is any better for what's coming than the current version. Indeed, the recommendation was to keep them the same but FDA seems to be going with LP.8.1. Might be replaced by descendants of Nimbus by then.

Here's a summary of Nimbus (NB.1.8.1) info:

theconversation.com/theres-a-new-c…