With dropping temperatures, a surge in respiratory illnesses is being experienced across the UK. This increase coincides with the emergence of two new strains of coronavirus, Nimbus and Stratus, each presenting distinct symptoms.
The UK has witnessed a notable uptick in confirmed Covid cases, rising from 7.6 per cent to 8.4 per cent within a week. The UK Health Security Agency (UKHSA) has issued a cautionary alert advising individuals exhibiting respiratory infection symptoms, like those of Covid-19, to refrain from contact with vulnerable populations and opt to stay home if feasible.
Recent reports indicate that XFG, or Stratus, and NB.1.8.1, known as Nimbus, are currently the dominant Covid strains. While various symptoms have been reported for each variant, they are not considered more perilous than earlier strains.
Key details about Stratus and Nimbus include:
Stratus, inclusive of two variants: XFG and XFG. 3, was initially detected in January 2025. It is believed to be highly transmissible, leading to a rise in cases across multiple regions.
The XFG. 3 variant accounted for approximately 40 per cent of Covid cases in the UK as of June, as reported by the UK Health Security Agency (UKHSA). Concurrently, the World Health Organization (WHO) notes that the XFG variant is currently the most prevalent strain in the UK.
Another strain under observation is NB. 1.8, or Nimbus. Similar to Stratus, Nimbus is thought to be highly transmissible but not more severe than other Omicron variants.
Initially identified in January 2025, Nimbus swiftly spread across Asia before surfacing in various countries globally, including the UK, the US, and Australia. In May 2025, the WHO categorized Nimbus as a “Variant Under Monitoring” (VUM), indicating potential public health risks.
Nimbus is the result of genetic recombination, comprising a mix of different Omicron subvariants. Studies suggest that this variant has a strong affinity for the ACE2 receptor, facilitating more efficient cell infiltration compared to earlier strains.
The primary initial symptom of the new Stratus strain is a hoarse voice. Medical experts have also identified additional symptoms such as nasal congestion, sore throats, digestive issues, and fatigue.
Dr. Nye, as reported by The Mirror, mentioned that individuals with this version typically experience severe throat pain and hoarseness, unlike the more general symptoms like cough, fever, and body aches.
Current data does not indicate that Stratus results in more severe illness or higher fatality rates than other circulating variants. Existing vaccines are expected to offer robust protection against severe illness and hospitalization.
One distinguishing feature of the Nimbus strain is an intense sore throat often described as a “razor blade sensation” upon swallowing. Nimbus has also been linked to increased instances of digestive symptoms such as nausea, diarrhea, bloating, and abdominal pain.
Available evidence suggests that Nimbus does not cause more severe disease compared to other prevalent variants. Current vaccines continue to provide effective defense against severe illness.
As of late September 2025, Stratus remains the prevailing strain in the UK, with Nimbus also present, according to the WHO.
The WHO highlighted that recent genomic surveillance classified 35 per cent of lineages as XFG. 3, 28 per cent as XFG, 11 per cent as NB.1.8.1, seven per cent as XFG.5, and seven per cent as XFG.3.4.1.
The WHO emphasized that virus mutations are common, and ongoing data analysis will enhance understanding of these variants’ interaction with the immune system. Vaccination for eligible individuals is crucial for safeguarding vulnerable populations and returning to a semblance of normalcy.
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