Overview
This article summarizes current knowledge on two new COVID-19 variants, NB.1.8.1 ("Nimbus") and XFG ("Stratus"), focusing on their spread, genetic makeup, impact, and public health relevance as of mid-2025.
Spread and Epidemiology
- NB.1.8.1 (Nimbus) has rapidly spread across Asia and globally, reaching 11% of sequenced cases by April 2025.
- XFG (Stratus) was first detected in Canada and now constitutes 25% of cases in Europe as of late May.
- Both variants are spreading quickly but show no evidence of causing more severe disease compared to other Omicron descendants.
Variant Naming and Background
- NB.1.8.1 received the nickname "Nimbus" due to its distinct lineage and potential for broader impact.
- XFG was nicknamed "Stratus" as both variants are currently competing for dominance.
- The group assigning names aims for terms that improve public communication and awareness.
Genetic Features and Mutation
- NB.1.8.1 is a recombinant variant from several Omicron offspring, carrying seven spike protein mutations and 20 additional changes elsewhere.
- XFG is from a recombination of LF.7 and LP.8.1.2, with four significant spike protein mutations.
Infectivity and Transmissibility
- NB.1.8.1 binds to the ACE2 receptor more strongly than most other variants and exhibits high infectivity in lab tests.
- Its increased ability to infect could explain its rapid rise, not immune escape.
Immune Evasion and Vaccine Efficacy
- XFG possesses mutations that improve its ability to evade some antibodies.
- Both variants show a moderate reduction in vaccine-induced antibody effectiveness, but vaccines, especially updated boosters, are still expected to protect against severe outcomes.
Symptoms and Clinical Observations
- Symptoms largely mirror those of Omicron, but NB.1.8.1 may feature a "razor blade" throat sensation and gastrointestinal symptoms.
- Hoarse voice reported among some recent cases in India.
Public Health Status and Monitoring
- NB.1.8.1 is classified by WHO as a Variant Under Monitoring (VUM), signaling attention but not major concern.
- XFG is not currently designated for additional WHO monitoring but is tracked via genomic surveillance.
Recommendations / Advice
- COVID-19 vaccination, including updated boosters, remains important for protection against severe illness with these variants.
- Monitor for shifts in symptoms, especially gastrointestinal complaints and sore throat, with new variants.
- Public health authorities continue to surveil these and other emerging variants for changes in risk.