A newly identified COVID-19 variant, NB.1.8.1—informally dubbed “Nimbus”—is rapidly gaining traction in the United States, prompting health officials to monitor its spread closely amid fears of a potential summer surge. Initially detected in Asia, the variant has now been reported in multiple U.S. states, raising questions about its transmissibility and impact on public health.
Rapid Spread and Global Presence
NB.1.8.1 is a subvariant of Omicron, specifically derived from the recombinant variant XDV.1.5.1. The World Health Organization (WHO) designated it as a “variant under monitoring” on May 23, 2025, due to its increasing global prevalence. As of mid-June, the variant has been detected in over 20 countries, including China, Hong Kong, Singapore, the United Kingdom, and several U.S. states such as California, Washington, Virginia, New York, Ohio, Rhode Island, and Hawaii .
In the U.S., NB.1.8.1’s presence is expanding swiftly. Recent data from the Centers for Disease Control and Prevention (CDC) estimate that the variant accounts for approximately 37% of COVID-19 cases, up from 15% just two weeks prior . This rapid increase suggests that NB.1.8.1 may soon overtake the currently dominant LP.8.1 variant.
Distinctive Symptoms and Clinical Observations
While NB.1.8.1 shares many symptoms with previous COVID-19 strains—such as fever, fatigue, cough, muscle aches, and nasal congestion—it has been notably associated with a particularly severe sore throat. Patients have described this symptom as akin to “swallowing razor blades,” leading some to refer to it as “razor blade throat” . Despite the alarming description, health experts emphasize that this symptom is not unique to NB.1.8.1 and can occur with other respiratory infections.
Transmissibility and Immune Evasion
Genomic analyses reveal that NB.1.8.1 possesses several spike protein mutations—specifically T22N, F59S, G184S, A435S, V445H, and T478I—that may enhance its ability to bind to human cells and evade immune responses . Laboratory studies indicate a 1.5 to 1.6-fold reduction in neutralization compared to the LP.8.1 variant, suggesting a modest increase in immune evasion. However, current data do not indicate that NB.1.8.1 causes more severe illness than other circulating variants.
Public Health Implications and Recommendations
The emergence of NB.1.8.1 comes at a time when COVID-19 testing and surveillance have declined, potentially obscuring the true extent of its spread. Health experts warn that the combination of waning immunity, reduced public health measures, and increased travel during the summer months could facilitate a surge in cases .
Despite the variant’s increased transmissibility, the WHO assesses the global public health risk posed by NB.1.8.1 as low. Existing COVID-19 vaccines are expected to remain effective in preventing severe disease caused by this variant . Nonetheless, experts recommend continued vigilance, including staying up to date with vaccinations, practicing good hand hygiene, wearing masks in crowded or high-risk settings, and seeking medical attention if symptoms develop.
As the situation evolves, public health authorities continue to monitor NB.1.8.1’s spread and impact, emphasizing the importance of preparedness and adherence to recommended precautions to mitigate potential outbreaks.
