New COVID-19 Strains ‘Stratos’ and ‘Nimbus’ Drive Rising Respiratory Illnesses This Winter
As temperatures drop and winter approaches, rates of respiratory diseases – including colds, influenza, and COVID-19 – are on the rise globally. Two new strains, dubbed “Stratos” and “Nimbus,” are currently the most prevalent variants, prompting health officials to closely monitor their spread and potential impact.
Stratos: The Dominant Strain in the UK
According to reports from the Mirror, the XFG sub-lineage, commonly referred to as Stratos, is currently the dominant COVID-19 strain in the United Kingdom. First identified in January 2025, Stratos and its derivatives, particularly Xfg.3, have become the primary source of infections since late September 2025. While the NB.1.8.1 strain, known as Nimbus, remains present, it is no longer the leading variant.
Identifying Stratos: Symptoms to Watch For
Experts indicate that a key indicator of a Stratos infection is a hoarse or coarse voice, often appearing as an early symptom. Beyond this distinctive sign, individuals infected with Stratos may experience:
- Sore throat
- Nasal congestion
- Nasal obstruction
- Digestive problems
- Extreme fatigue
Currently, data suggests that Stratos does not cause more severe illness than other circulating mutations. However, some health professionals believe it may be more adept at evading immunity gained from prior infections or vaccinations.
Nimbus: A Strain with Enhanced Infectivity
Research suggests that Nimbus (NB.1.8.1) exhibits a strong affinity for ACE2 receptors, potentially increasing its ability to infect cells compared to previous strains. A particularly notable symptom associated with Nimbus is a severe sore throat, often described as a “razor sensation” during swallowing.
In addition to the intense sore throat, individuals infected with Nimbus may also experience:
- Nausea
- Diarrhea
- Swelling
- Abdominal pain
Nimbus is a genetic mosaic, comprised of various Omicron sub-lineages. Despite its enhanced infectivity, current data indicates that NB.1.8.1 does not cause more serious disease, and existing vaccines continue to offer effective protection against severe illness. First monitored in January 2025, Nimbus rapidly spread across Asia before being detected in countries including the United Kingdom, the United States, and Australia.
A History of COVID-19 Variants
Understanding the evolution of COVID-19 requires recognizing the major strains that have emerged since the start of the pandemic:
- Alpha (B.1.1.7): First discovered in the UK in late 2020.
- Beta (B.1.351): First reported in South Africa in late 2020.
- Gamma (P.1): First discovered among travelers from Brazil in early 2021.
- Delta (B.1.617.2): First reported in India in late 2020, becoming the globally dominant strain until the emergence of Omicron.
- Omicron (B.1.1.1.529): First reported in South Africa in late 2021, and its descendants have dominated globally since.
Key Omicron sub-variants include:
- BA.1, Ba.2, Ba.3, Ba.4, Ba.5: These early sub-strains drove infection waves throughout 2022.
- BA.2.86 (Perola): Characterized by a large number of mutations, requiring close monitoring in 2023.
- EG.5 (ERIS): A dominant strain in 2023.
- JN.1: A highly widespread sub-lineage prominent in late 2023 and early 2024.
- KP.2 (FLIRT): A JN.1 sub-variant, part of the “Flirt” family prominent in mid-2024.
- XEC (Stratus): Became the globally dominant strain in early 2025.
Preventing the Spread: Reaffirming Protective Measures
If you experience symptoms of a respiratory infection, such as COVID-19, or develop a fever, it is advisable to stay home and avoid contact with others. For those who must venture out, the established guidelines for coexisting with COVID-19 remain effective in reducing transmission:
- Wear a well-fitting, multi-layered face covering or surgical mask.
- Avoid crowded spaces, including public transportation and poorly ventilated indoor areas.
- Prioritize outdoor activities when possible, maintaining distance from others.
- Cover your mouth and nose when coughing or sneezing.
- Wash your hands frequently with soap and water for at least 20 seconds, or use hand sanitizer, especially after coughing, sneezing, or touching your face.
