WHO tracking Omicron sub-variant NB.1.8.1 as cases rise in the US, UK and parts of Asia

A newly emerging strain of Covid-19, officially designated NB.1.8.1 and nicknamed Nimbus, is rapidly gaining attention from health authorities around the world. Identified as a descendant of the Omicron lineage, the World Health Organization (WHO) has placed this variant under its close observation list as infections surge in multiple regions.

Detected in the US, UK, and parts of Asia

The Global Initiative on Sharing All Influenza Data (GISAID) reports that NB.1.8.1 has been identified in at least 13 U.S. states, including New York, California, New Jersey, and Illinois. Meanwhile, in the United Kingdom, health officials have recorded a 10% increase in Covid-related hospital admissions, with 947 individuals hospitalized in the week ending May 31, according to the UK Health Security Agency (UKHSA).

Asia is also witnessing a rise in infections. Countries such as China, Singapore, Thailand, and India are seeing spikes in cases. While India has not yet confirmed cases specifically attributed to the Nimbus strain, Covid-related deaths in the country surpassed 100 on Monday, prompting heightened alert among public health authorities.

What is the Nimbus variant?

NB.1.8.1 is a sub-lineage of Omicron, the dominant strain responsible for previous global surges. Since it was first identified in January 2025, Nimbus has been spreading at a steady pace and is currently among six variants under WHO surveillance.

Symptoms: mostly mild, but “razor blade throat” reported

Most of the symptoms resemble those seen in previous Omicron sub-variants, including:

• Cough
• Fever
• Fatigue
• Muscle aches
• Congestion
• Headache
• Nausea or vomiting
• Loss of smell or taste

Notably, some patients have described experiencing an unusually severe sore throat symptom, termed “razor blade throat,” which has drawn concern for its intensity.

Dr. Ankita Baidya, head of infectious diseases at Manipal Hospital, Dwarka, commented on the symptoms: “This time we see a sense of loss of taste and smell along with normal viral-like symptoms like fever, cough, and cold, and other flu-like symptoms like sore throat and running nose. Headache and body ache is also noted.”

Is Nimbus a cause for concern?

Despite its rapid spread, the WHO currently assesses the public health risk of NB.1.8.1 as “low.” The agency states that existing Covid-19 vaccines are still expected to offer strong protection against symptomatic and severe disease caused by the variant.

Between April 21 and 27, Nimbus made up 10.7% of all sequenced Covid cases globally, with data drawn from 518 genome samples across 22 countries.

According to Dr. Baidya, “Nimbus has been a fast-spreading strain since January 2025… though it’s spreading fast among people, it’s not that deadlier as such, causing mild to moderate disease only.”

A second variant also under watch

Alongside Nimbus, another new variant — XFG, unofficially called Stratus — has also sparked concern. Although not officially tracked by WHO at this stage, Stratus has shown signs of rapid transmission in some regions.

“Stratus is again under scrutiny,” said Dr. Baidya, noting that while cases remain fewer compared to Nimbus, both strains are currently among the most actively circulating.

“Nimbus is under monitoring according to WHO, seeing how the pattern of disease and its spread is being studied simultaneously,” she added.

As the global health community watches these developments closely, experts continue to urge vaccination, vigilance, and early testing for those exhibiting symptoms, especially amid signs of a summer resurgence.

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