A new subvariant of SARS-CoV-2, designated NB.1.8.1, is rising in prevalence across multiple regions, according to a May 2025 evaluation from the World Health Organization.
Though still classified only as a "variant under monitoring" (VUM), the data show a steady, global uptick-particularly in the Western Pacific and European regions.
It carries mutations associated with both increased infectivity and partial immune escape. But so far, no increase in disease severity has been observed.
The WHO isn’t sounding an alarm-but it’s watching closely.
📈 What’s New About NB.1.8.1?
NB.1.8.1 is a descendant of the recombinant variant XDV.1.5.1 and was first sampled in January 2025. It’s defined by several spike mutations, including:
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T22N, F59S, G184S, A435S, V445H, T478I
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Additional differences from JN.1 include L455S, F456L, Q493E
Some of these mutations (notably A435S and T478I) are known to impact:
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Antibody evasion, especially Class 1/2 and Class 1/4 monoclonal antibodies
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hACE2 receptor binding affinity, potentially increasing transmissibility
Still, the WHO’s preliminary assessment categorizes the overall public health risk as low.
🌍 Where It’s Spreading
As of mid-May 2025:
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518 NB.1.8.1 sequences were logged across 22 countries
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The variant made up 10.7% of all global sequences in epidemiological week 17 (April 21–27)
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That’s up from just 2.5% in week 14
Notably, prevalence grew in:
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The Western Pacific Region: from 8.9% to 11.7%
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The Region of the Americas: 1.6% to 4.9%
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The European Region: 1.0% to 6.0%
No sequences have yet been identified from Africa or the Eastern Mediterranean.
💉 Vaccine Impact and Immune Evasion
So far, data from pseudovirus neutralization tests show:
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A 1.5–1.6x reduction in neutralization compared to LP.8.1.1
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Antigenic behavior similar to other JN.1 sublineages
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No clear evidence of major breakthrough risk
Lab studies using sera from mice vaccinated with current formulations (LP.8.1 or KP.2-based mRNA vaccines) produced neutralizing responses comparable to those seen with other recent variants.
WHO’s vaccine advisory group (TAG-CO-VAC) continues to recommend monovalent JN.1 or KP.2 vaccines for 2025, stating LP.8.1 remains a suitable alternative.
🏥 Clinical Severity: No Change Yet
NB.1.8.1 has not shown signs of:
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Higher hospitalization rates relative to infections
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Increased ICU admissions or all-cause mortality
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Resistance to key antivirals like Paxlovid (Nirmatrelvir)
That said, WHO notes a drop in data reporting from several countries, meaning trends in severe cases should be interpreted with caution.
The variant does carry a polymerase mutation (D284Y), but its impact on drugs like Remdesivir or Molnupiravir is still unstudied.
🕳️ Watching, Not Panicking
Despite its fast growth and concerning mutations, NB.1.8.1 hasn’t triggered a new global alert. It hasn’t met the threshold for being declared a Variant of Interest (VOI), and WHO’s standing risk rating remains:
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Growth advantage: Moderate risk, low confidence
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Immune escape: Low risk, low confidence
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Severity: Low risk, low confidence
This is the pattern with many recent COVID variants: fast-moving, mildly evasive, but not necessarily more dangerous.
For now, it’s under monitoring. For now, vaccines still work. For now, the risk is manageable.
But the data-and the virus-keep moving.
