COVID-19 Variant NB.1.8.1 (“Nimbus”): Global Spread and Characteristics
COVID-19 Variant NB.1.8.1 (“Nimbus”): Global Spread and Characteristics
A new COVID-19 variant, NB.1.8.1, also known as “Nimbus,” is spreading globally. As of mid-May 2025, it comprised nearly 11% of sequenced samples reported worldwide. In the United States, it accounted for approximately one-third of COVID-19 cases. The variant has been detected in travelers arriving in the U.S. from the eastern Mediterranean, Southeast Asia, and western Pacific regions, with cases confirmed in California, Washington state, Virginia, and New York.
First identified in early 2025, Nimbus is a subvariant of Omicron. Early data suggest that it may be more transmissible than other circulating variants due to mutations in its spike protein. However, current data from the World Health Organization (WHO) indicates that Nimbus does not appear to cause more severe illness than previous variants. Reports of increased hospitalizations or deaths linked to Nimbus have not been observed in routine surveillance.
While the WHO has designated Nimbus as a “variant under monitoring” and considers the global public health risk low, some regions have reported increases in COVID-19 cases and hospitalizations. The variant’s prevalence has fluctuated; at one point, it surpassed LP.8.1 to become the most detected variant globally, before LP.8.1’s prevalence slightly increased again.
A distinctive symptom associated with Nimbus is a severe sore throat described as a “razor blade throat,” characterized by sharp pain when swallowing. However, experts emphasize that this symptom is anecdotal and there is no conclusive evidence that Nimbus causes more severe sore throats than other variants. Other common symptoms reported include fever, cough, fatigue, muscle aches, and gastrointestinal symptoms such as diarrhea, constipation, and nausea. These symptoms align with those of typical Omicron subvariants.
The WHO and other experts state that current COVID-19 vaccines are expected to remain effective against Nimbus and offer protection against severe illness. Preventive measures remain the same as with previous variants: handwashing, avoiding contact with sick individuals, ensuring good ventilation, vaccination, and masking in high-risk settings.
Experts advise that individuals experiencing severe sore throats should consult a doctor to rule out other conditions, such as strep throat, a bacterial infection requiring antibiotic treatment. Strep throat can lead to complications including peritonsillar abscesses, rheumatic fever, post-streptococcal glomerulonephritis, and, in rare cases, streptococcal toxic shock syndrome or meningitis.
The CDC recommends COVID-19 vaccination for most adults 18 and older to maintain protection against severe illness, hospitalization, and death. Older adults, high-risk individuals, and those who have never been vaccinated are particularly encouraged to get vaccinated. Parents should consult healthcare professionals for children under 18. The CDC also notes that immunity from previous vaccines wanes over time, highlighting the importance of updated vaccinations.
In conclusion, while Nimbus is a more transmissible variant, current evidence does not suggest it causes more severe illness than previous COVID-19 variants. The WHO continues to monitor the situation, and public health recommendations remain consistent with those for other Omicron subvariants.
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