As the autumn and winter seasons approach, the colder weather brings an increase in respiratory illnesses such as the common cold, flu, and coronavirus. Currently, two strains of COVID-19 are in the spotlight for being the most prevalent variants.
The latest strain identified by the World Health Organisation (WHO) is the XFG variant, also known as Stratus, which has become the dominant COVID-19 strain in the UK since its detection in January 2025. Another strain under observation is NB.1.8.1, referred to as Nimbus, which shares similarities with Stratus in terms of contagiousness but does not appear to cause more severe symptoms than other Omicron variants.
Recent genomic surveillance by the WHO revealed that XFG.3 accounted for 35% of the cases, followed by XFG at 28%, NB.1.8.1 at 11%, XFG.5 at 7%, and XFG.3.4.1 at 7%. The WHO emphasizes that these variants do not seem to result in more severe illnesses compared to other circulating strains.
The WHO emphasizes the normalcy of virus mutations and the importance of ongoing research to understand how these variants interact with our immune systems. Vaccination remains crucial for protection, and timely vaccination is recommended for eligible individuals.
In the UK as of late September 2025, the primary COVID-19 strain is Stratus, specifically the XFG subvariant and its derivatives, notably XFG.3. While Nimbus (NB.1.8.1) is still present, it is no longer the predominant strain.
Experts highlight that viruses continue to evolve, and vigilance is necessary as new variants emerge. Stratus is known for potentially causing a hoarse voice as a common symptom, indicating early infection.
Although Stratus is believed to spread easily, current information does not suggest that it leads to more severe illness or increased fatality compared to other variants. Vaccines are expected to provide effective protection against severe outcomes, with the global public health risk posed by Stratus evaluated as “low” by the WHO.
Research indicates that NB.1.8.1 has a higher affinity for infecting cells due to its interaction with the ACE2 receptor. This strain is linked to symptoms like a severe sore throat and gastrointestinal issues. However, it does not appear to cause more severe illnesses than other strains, and existing vaccines remain effective.
Nimbus, categorized as a “Variant Under Monitoring” by the WHO, emerged in January 2025 and spread across multiple countries. It was identified in regions like the UK, US, and Australia, becoming a prominent strain in some areas.
For individuals experiencing respiratory infection symptoms, it is advisable to stay home if feeling unwell to prevent spreading the infection, especially to vulnerable individuals. Adhering to public health guidelines can help mitigate the risk of transmission.
Vaccination remains a vital defense against severe illness from flu and COVID-19, as emphasized by the WHO. Eligible individuals are encouraged to receive their vaccinations as scheduled to enhance protection against the evolving variants.