WHO's latest Stratus and Nimbus Covid strain differences and symptoms - full list

With the weather turning colder as we head into autumn and winter, the rates of respiratory illnesses start to rise - including the common cold, flu and coronavirus. Currently, there are two strains of COVID that are making headlines as they are becoming the latest variants with the highest prevalence.

According to the World Health Organisation (WHO), the XFG variant is the latest COVID-19 strain and currently the most prevalent in the UK. It was first detected in January 2025, and it is more commonly referred to as Stratus.

Stratus is the latest addition, but another strain currently "under monitoring" is NB.1.8.1, which is otherwise known as Nimbus. Just like Stratus, it's thought to be very contagious, though it doesn't seem to be any more severe than other Omicron variants.

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The WHO says: "Our genomic surveillance of SARS-CoV-2 lineages last month found that 35% were classified as XFG.3, 28% were classified as XFG, 11% were classified as NB.1.8.1, 7% were classified as XFG.5 and 7% were classified as XFG.3.4.1."

You can see the WHO's latest data on its National flu and COVID-19 surveillance reports. Current WHO data does not indicate that these variants lead to more severe illness than other variants in circulation.

The WHO adds: "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."

As of late September 2025, the WHO says the primary COVID-19 strain in the UK is the Stratus variant, which refers to the Omicron subvariant XFG and its derivatives, especially XFG.3. The Nimbus variant (NB.1.8.1) is still around, but it’s not the leading strain anymore.

What is Stratus and its symptoms?

It's important to mention that the virus keeps mutating, and this information might change as new variants emerge and some fade away. Regarding the key characteristics of Stratus and its most common symptoms, many experts say that the main sign to look out for is a hoarse or raspy voice, which can show up as an early sign of infection.

Stratus is thought to spread quite easily, which has led to an increase in cases in various areas. In addition to a hoarse voice, you might also experience:

• sore throat
• nasal congestion
• blocked nose
• digestive problems
• feeling really tired

The latest information doesn’t indicate that Stratus causes more severe illness or is deadlier than other variants currently circulating. Some health professionals believe that Stratus may be better able to evade immunity from past infections or vaccinations.

Still, current vaccines are expected to offer solid protection against severe illness and hospitalisation. The WHO has evaluated the extra public health risk that Stratus poses as "low" on a global scale.

What is Nimbus and its symptoms?

Research indicates that NB.1.8.1 has a strong affinity for the ACE2 receptor, which means it can infect cells more effectively than previous strains. Numerous doctors have noted that a distinctive symptom of this strain is an extremely painful sore throat, often referred to as a "razor blade sensation" during swallowing.

This variant has also been linked to a greater occurrence of gastrointestinal symptoms, including nausea, diarrhoea, bloating, and abdominal pain. Nimbus is formed through genetic recombination, making it a mix of various Omicron subvariants.

Current data suggest that NB.1.8.1 does not lead to more severe illness compared to other circulating variants. The existing vaccines still provide effective protection against serious disease.

In May 2025, the WHO labelled Nimbus a "Variant Under Monitoring" (VUM), a category for variants that could threaten public health. Nimbus was first spotted in January 2025 and rapidly made its way through Asia before being found in various countries worldwide, such as the UK, US, and Australia. It became one of the leading strains in some areas.

The names of every major COVID-19 strain

• Alpha (B.1.1.7): First detected in the UK in late 2020.
• Beta (B.1.351): First reported in South Africa in late 2020.
• Gamma (P.1): First detected in travellers from Brazil in early 2021.
• Delta (B.1.617.2): First reported in India in late 2020 and became the dominant strain worldwide until Omicron.
• Omicron (B.1.1.529): First reported in South Africa in late 2021, this lineage and its subvariants have dominated globally ever since.

Major Omicron subvariants

• BA.1, BA.2, BA.3, BA.4, BA.5: These early sub-lineages drove infection waves throughout 2022.
• BA.2.86 (Pirola): Noted for its high number of mutations, prompting close monitoring in 2023.
• EG.5 (Eris): A dominant strain in 2023.
• JN.1: A fast-spreading Omicron subvariant prominent in late 2023 and early 2024.
• KP.2 (FLiRT): A subvariant of JN.1, part of the "FLiRT" family of variants prominent in mid-2024.
• XEC (Stratus): Became the dominant global strain in early 2025.

What should people who think they have COVID do?

If you're showing signs of a respiratory infection like COVID-19, and you're running a fever or just don't feel up to going to work or doing your usual activities, it's best to steer clear of vulnerable people and stay home if you can. For those who really can't stay home, the Living with COVID guidelines are still the same, and they can help lower the risk of spreading your infection to others:

• Make sure to wear a snug face covering with multiple layers or a surgical mask.
• Stay away from crowded spots like public transport, big social events, or any enclosed areas that aren't well-ventilated.
• Try to do your exercise outside in places where you won't be close to others.
• Always cover your mouth and nose when you cough or sneeze
• Wash your hands often with soap and water for 20 seconds or use hand sanitiser after coughing, sneezing, or blowing your nose, and before eating or handling food and avoid touching your face.

Are there still COVID vaccines?

The WHO says: "Vaccines remain our best defence against severe disease and hospitalisation from flu and COVID-19." You can find details of who is eligible and how they can book their vaccination here.