Two new Omicron subvariants, NB.1.8.1 (“Nimbus”) and XFG (“Stratus”), are spreading rapidly worldwide. Here’s what we know so far.
They are spreading fast
NB.1.8.1 was first identified in January 2025 and rose from 2.5 % to 11 % of sequenced cases in just one month.
XFG spread even faster, making up 25 % of cases in Europe by late May. However, neither variant appears to cause more severe disease than other Omicron lineages.
The name “Nimbus” is not from Harry Potter
The nickname “Nimbus” was given by researchers and citizen scientists who name variants to improve public communication.
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The name refers to a type of cloud, not Harry Potter or Futurama. “Stratus” was chosen because the two variants are competing for global dominance.
They are products of genetic recombination
Nimbus emerged through recombination of several Omicron lineages, carrying seven new spike protein mutations and 20 additional changes.
Stratus is a mix of LF.7 and LP.8.1.2, with four key spike mutations.
Nimbus binds strongly to human cells
Studies show Nimbus attaches more effectively to the ACE2 receptor than many other variants, making it highly infectious.
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This may explain why it is spreading so efficiently.
Stratus is better at evading the immune system
Stratus mutations help it evade antibodies, making it harder for the immune system to neutralize.
However, it binds less strongly to the ACE2 receptor, so it may spread less effectively unless it accumulates more mutations.
Vaccines still provide protection
Lab tests show only a modest reduction in antibody effectiveness against these variants.
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Vaccination, particularly with updated boosters, is still expected to protect against severe illness, hospitalization, and death.
Some possible new symptoms to watch for
While symptoms are generally similar to other Omicron variants, reports suggest Nimbus may cause a “razor blade” sensation in the throat and gastrointestinal issues such as nausea, vomiting, diarrhea, bloating, or stomach pain.
Doctors in India have also reported hoarseness as a common symptom.
Nimbus is closely monitored by WHO
The World Health Organization (WHO) has classified Nimbus as a Variant Under Monitoring (VUM).
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This means it is noteworthy but not currently considered a major threat. Stratus has not received this designation but is under routine genomic surveillance.
They could trigger new waves of infections
Both variants have shown potential to drive case surges in different regions. Experts warn that either Nimbus or Stratus could become the next dominant variant globally.
Recombinant origins make them unpredictable
Because they were formed through recombination, these variants may continue to evolve rapidly, potentially gaining mutations that improve spread or immune evasion.
Overall risk is currently considered low
Despite their rapid spread, WHO currently assesses the overall public health risk of Nimbus as low.
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This means they are not associated with greater severity or higher mortality compared to other Omicron descendants.
This article is based on information from Gavi.org.