Rising Cases of New COVID Variants Stratus and Nimbus in USA and UK: What You Need to Know

In 2025, two new COVID-19 variants, Stratus (XFG) and Nimbus (NB.1.8.1), have been pushing an evident increase in COVID cases across the United States and the United Kingdom. These variants, both descendants of the Omicron family, carry mutations that enhance their transmissibility and allow them to evade immune defenses, leading to widespread infections. This article explores the latest scientific insights, symptoms, spread, and safety measures associated with these new variants.

What Are the Stratus and Nimbus Variants?

Stratus and Nimbus are recombinant Omicron sub-lineages identified in early 2025. Stratus (XFG) originated in Southeast Asia and quickly spread globally, becoming the dominant strain in many regions. Nimbus (NB.1.8.1) was first identified in China and has similarly spread worldwide.

Stratus (XFG): A hybrid of two Omicron-related variants (LF.7 and LP.8.1.2), exhibiting increased ability to evade immune responses.

Nimbus (NB.1.8.1): Known for its efficient cell attachment, making it highly transmissible but not more severe in illness.

In the USA, the Centers for Disease Control and Prevention (CDC) recognize that the Stratus (XFG) variant is currently dominant in many states. While it spreads more easily and can partially evade immunity, CDC officials confirm that there is no current evidence of increased hospitalization or mortality rates associated with Stratus.  Experts like Dr. Rishi Khakhkhar, Chief Medical Officer at Counsel Health, observe that COVID-19 is settling into a more seasonal respiratory virus pattern. He highlights that although infections may surge with these new variants, population immunity from vaccines and prior infections provides strong protection against severe disease and hospitalization. In the UK, the UK Health Security Agency (UKHSA) states that while Stratus and Nimbus substantially account for recent COVID cases; current data does not show more severe illness or increased death rates from these variants.

Current Situation in the USA

In the United States, Stratus has overtaken Nimbus as the dominant COVID-19 variant, accounting for approximately 79-85% of cases in many states, especially in the western parts and urban centers. Wastewater surveillance data indicate high to very high viral levels in 19 states. The rise in cases coincides with increased social interactions, travel, and relatively low vaccine uptake during summer and autumn. Despite higher transmissibility, the severity of illness caused by Stratus is comparable to previous Omicron subvariants. Updated vaccines targeting these variants are available and recommended as preventive measures.

Current Situation in the UK

The UK is experiencing a surge mainly driven by Stratus (about 63% of monitored cases) followed by Nimbus (approx. 25%). Health authorities in the UK advise vigilance but report that these variants have not increased the severity of disease. Both variants predominantly cause mild to moderate symptoms. UK Health Security Agency emphasizes self-isolation and avoiding vulnerable contacts when infected. Stratus may sometimes cause a hoarse voice and Nimbus frequently produces a sharp “razor-blade” sore throat, intense fatigue, mild cough, and digestive issues.