While COVID-19 might not dominate headlines as it once did, the virus continues to evolve. Health authorities and researchers worldwide remain vigilant, tracking new versions of SARS-CoV-2 that emerge and spread. Two such variants recently garnering attention are unofficially nicknamed “Nimbus” and “Stratus.”
Here’s what you need to understand about NB.1.8.1 (Nimbus) and XFG (Stratus), the latest sub-variants competing for global prevalence.
What Are NB.1.8.1 (“Nimbus”) and XFG (“Stratus”)?
These are relatively new sub-variants of the Omicron lineage. NB.1.8.1, dubbed “Nimbus,” has been designated a Variant Under Monitoring (VUM) by the World Health Organization (WHO), indicating it warrants close tracking due to its spread and potential public health implications. XFG, informally called “Stratus,” is also being watched closely by researchers, although it hasn’t received the same WHO designation yet.
Why the Unofficial Nicknames?
The informal names “Nimbus” and “Stratus” come from a group of researchers and citizen scientists who previously coined terms like Kraken, Centaurus, and Eris. According to Prof T. Ryan Gregory, part of this group, they assign nicknames to variants they believe will become widely discussed. NB.1.8.1 (“Nimbus”) was the first variant to receive such a nickname in nearly two years after the prominence of the “Pirola” lineage.
The decision to name both Nimbus and Stratus reflects their observed rapid spread and competition for global dominance. The names themselves are drawn from meteorological terms, referencing cloud types – a departure from earlier mythological or astronomical themes used for other variants. Gregory notes the “n” and “b” in Nimbus were also helpful coincidences for NB.1.8.1.
Fast-Spreading “Frankenstein” Variants
Both Nimbus and Stratus have demonstrated rapid global spread. NB.1.8.1, identified in January 2025, quickly increased its share of sequenced cases in Asia and other regions, accounting for 11% globally by late April 2025.
XFG, first detected in Canada, has also spread quickly, notably in Europe where it comprised 25% of cases by late May, and is also rising in India. The WHO’s risk evaluation for NB.1.8.1 even noted that XFG appeared to be spreading faster than Nimbus at that time.
Interestingly, both variants are products of genetic recombination, sometimes referred to as “Frankenstein” variants. This occurs when different virus versions infect the same cell and swap genetic material.
NB.1.8.1 (Nimbus) is a hybrid descended from XDV (itself a hybrid of JN.1 and XDE). It possesses seven new mutations in its spike protein and 20 other genetic changes.
XFG (Stratus) is a mix of two earlier variants, LF.7 and LP.8.1.2, featuring four key mutations in its spike protein.
Are They Causing More Severe Disease?
Despite their rapid spread, current evidence does not suggest that NB.1.8.1 or XFG are associated with more severe illness compared to other circulating Omicron variants. The WHO’s Technical Advisory Group on Virus Evolution stated that the available data for NB.1.8.1 does not indicate additional public health risks relative to other prevalent Omicron lineages. While case and hospitalization increases have occurred in some areas alongside the rise of these variants, this isn’t proof of increased severity per infection.
Key Differences: Infectivity vs. Immune Evasion
Lab studies highlight functional differences between the two:
Nimbus (NB.1.8.1): Shows strong binding to the ACE2 receptor (the entry point into human cells) and high infectivity in lab-grown cells. Its infectivity may be significantly higher than some currently dominant variants, potentially explaining its rapid takeover in some areas due to efficient infection rather than superior immune evasion.
Stratus (XFG): Appears better at evading antibodies. Two of its spike mutations may help it dodge immune responses, including potentially a broader class of antibodies. However, it doesn’t bind to the ACE2 receptor as strongly as some other variants, suggesting its strength lies more in immune escape than inherent infectivity if it were isolated.
What About Vaccine Effectiveness?
Laboratory tests using blood samples from vaccinated or previously infected individuals showed some reduction in antibody effectiveness against both variants compared to the most common global variant. XFG showed roughly a 2-fold drop, while NB.1.8.1 showed about a 1.6-fold reduction.
However, experts emphasize that vaccines, particularly updated boosters, are still likely to provide significant protection against severe illness, hospitalization, and death from these variants. While they might be slightly better at evading antibodies, the core protection against severe outcomes from vaccination remains important.
Are There Specific Symptoms?
While symptoms for both variants generally resemble those of earlier Omicron versions, some specific anecdotal observations have been reported by doctors in certain regions:
Nimbus (NB.1.8.1): UK doctors noted potential signs like a “razor blade” sensation in the throat and gastrointestinal symptoms (nausea, vomiting, diarrhea, bloating, etc.). These observations are not yet confirmed by scientific studies.
General Observations (India): Doctors in India have flagged a hoarse voice as common among patients recently presenting with COVID-19, coinciding with the rise of variants like XFG there.
It’s crucial to remember these are preliminary observations and not definitive, scientifically validated symptom profiles.
How Closely Are They Being Monitored?
As mentioned, NB.1.8.1 is a WHO Variant Under Monitoring (VUM), signaling that it’s noteworthy and warrants prioritized tracking, but it’s not currently classified as a Variant of Interest or Concern, which would indicate a higher level of public health risk. The overall risk assessment for NB.1.8.1 by WHO is currently considered “low.”
XFG is not currently a VUM but remains under routine genomic surveillance like all other emerging variants. Health authorities worldwide continue to track sequences for any signs of concerning changes in spread, mutations, or potential impact on public health.
In summary, Nimbus and Stratus represent the ongoing evolution of the COVID-19 virus. While spreading quickly due to factors like infectivity or immune evasion, current data suggests they are not causing more severe disease than other circulating strains. Vaccination, especially with updated boosters, remains a critical tool for protection against severe outcomes, and monitoring continues worldwide.
