Two variants, Stratus and Nimbus, are currently the most prevalent strains of Covid across the UK as cases surge – but they both present varying symptoms and have key differences
As temperatures drop, many people have started to feel unwell with respiratory illness rates surging across the UK. This comes as two new strains of coronavirus, Nimbus and Stratus, have been making the rounds with varying symptoms.
There has been a significant rise in confirmed Covid cases across the UK, with data revealing that in the space of a week, it surged from 7.6 per cent to 8.4 per cent. This has seen the UK Health Security Agency (UKHSA) issue a warning, urging those with symptoms of a respiratory infection such as Covid-19 to “avoid contact” with vulnerable individuals and stay home if possible.
Official reports show that XFG, also known as Stratus, and NB.1.8.1, known as Nimbus, are now the most prevalent strains of Covid. A variety of symptoms have been reported for each variant, but they aren’t said to be any more dangerous than previous strains.
Here’s everything you need to know, including the key differences between Stratus and Nimbus…
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What is the Stratus variant?
Stratus, which includes two variants: XFG and XFG. 3, was first detected in January 2025. It is believed to transmit rather easily, which has resulted in rising cases across different regions.
The XFG. 3 variant made up around 40 per cent of Covid cases in the UK as of June, according to the UK Health Security Agency (UKHSA). Meanwhile, the World Health Organization (WHO) states that the XFG variant is currently most widespread across the UK.
What is the Nimbus variant?
Another strain “under monitoring” is NB. 1.8, also known as Nimbus. Similar to Stratus, it’s believed to be highly transmissible, although it doesn’t appear to be more serious than other Omicron variants.
It was initially detected in January 2025 and swiftly spread across Asia before appearing in numerous nations globally, including the UK, the US, and Australia. In May 2025, the WHO designated Nimbus a “Variant Under Monitoring” (VUM), a classification for variants that might pose public health risks.
Nimbus is formed through genetic recombination, making it a mix of various Omicron subvariants. Studies suggest that the variant is highly attracted to the ACE2 receptor, enabling it to infiltrate cells more efficiently than earlier variants.
Stratus Symptoms
The most common initial symptom of the new Stratus strain is a hoarse or raspy voice. Medical experts have also noted additional Stratus symptoms, including blocked noses, nasal congestion, sore throats, digestive issues and exhaustion.
Dr Nye told The Mirror: “In most ways, it’s almost identical. However, people with this version tend to get very sore throats and a hoarse voice, compared with the other more general symptoms like cough, temperature and aches.”
Current data doesn’t suggest that Stratus triggers more serious illness or proves more lethal than other variants presently in circulation. Existing jabs are anticipated to provide robust defence against serious illness and hospital admission.
Nimbus symptoms
Doctors have observed that a characteristic sign of the Nimbus strain is an excruciating sore throat, frequently described as a “razor blade sensation” when swallowing. Nimbus has also been associated with increased instances of digestive symptoms, such as nausea, diarrhoea, bloating, and stomach pain.
Present evidence suggests that Nimbus doesn’t cause more serious disease compared to other circulating variants. Current vaccines continue to deliver effective protection against severe illness.
Which variant is the most predominant?
When the variants are compared, Stratus is currently the most dominant strain across the UK, while the Nimbus variant remains present, according to the WHO as of late September 2025.
The WHO explained: “Our genomic surveillance of SARS-CoV-2 lineages last month found that 35 per cent were classified as XFG.3, 28 per cent were classified as XFG, 11 per cent were classified as NB.1.8.1, seven per cent were classified as XFG.5 and seven per cent were classified as XFG.3.4.1.”
The WHO also shared: “It’s normal for viruses to mutate and change, and as more data becomes available on these variants we’ll have a better understanding of how they interact with our immune systems and how to optimise our protection, as well as actions we can take to keep the most vulnerable safe and live our lives as normally as possible. The most important thing is for those eligible to get their vaccination when it is due.”