COVID-19 may have disappeared from the headlines, but it has neither disappeared nor stopped evolving — and the emergence of a New COVID Variant proves it.
The World Health Organization (WHO) currently lists six “variants under monitoring” — variants whose increasing prevalence and potential implications for public health warrant increased attention and monitoring by health authorities.
Among them is NB.1.8.1, unofficially nicknamed “Nimbus” by a group of researchers and citizen scientists, who are already responsible for the unofficial names Kraken, Centaurus, and Eris — names intended to make the variants more recognizable to the general public.
Another variant is also attracting the attention of observers, although it has not yet been classified as a variant under monitoring by the WHO: this is XFG, nicknamed “Stratus.”
Here is what we know so far about these two variants.
New COVID Variant: They spread quickly — but are not necessarily more dangerous
Since its identification in January 2025, NB.1.8.1 has spread rapidly in Asia and then to other regions, accounting for 11% of sequenced cases at the end of April 2025, compared to only 2.5% a month earlier.
This is rapid progression compared to other variants in circulation. Although an increase in cases and hospitalizations has been reported in some countries—particularly in the Western Pacific region—there is no evidence to suggest that NB.1.8.1 causes a more severe form of the disease than other Omicron descendants.
In its latest risk assessment, published on May 23, 2025, the WHO Technical Advisory Group on Virus Evolution stated: “The available data on NB.1.8.1 do not suggest any additional risk to public health compared to other Omicron sublineages currently circulating.”
XFG, meanwhile, was first detected in Canada and is also spreading rapidly worldwide. By the end of May, it accounted for 25% of cases in Europe, compared with 9% for NB.1.8.1. It is also gaining momentum in India, where 206 cases had been detected as of June 11.
In its risk assessment of NB.1.8.1, the WHO noted that XFG was spreading even faster than that variant. Again, there is currently no evidence to suggest that it is associated with a more severe form of the disease.
New COVID Variant : “Nimbus” is not a reference to Harry Potter
According to Professor T. Ryan Gregory of the University of Guelph in Canada—a member of a group of researchers and citizen scientists who assign unofficial nicknames to COVID variants likely to attract public attention—NB.1.8.1, aka “Nimbus,” is the first variant to receive such a nickname since “Pirola ” (BA.2.86) in August 2023.
“Since then, it’s been all about Pirola, with almost only direct descendants or recombinations of its descendants circulating,” he told VaccinesWork.
He explains that several factors justified the choice of a new nickname for NB.1.8.1: “It’s not just another member of the Pirola variant soup: it has emerged as a distinct lineage, which is now evolving in human hosts. It has caused waves in certain regions of the world—and could do so more widely. It was also classified as a variant under monitoring (VUM) by the WHO on May 23.”
Some might think that the name Nimbus refers to the Nimbus 2000 magic broomstick in Harry Potter, the Nimbus spaceship in the Futurama series, or the character Mr. Nimbus in Rick and Morty. But Gregory clarifies that the name refers to a type of cloud, adding, “And it’s convenient: the word contains the letters n and b, like NB.1.8.1.”
The decision to also assign a nickname to the XFG variant stems from the observation that Nimbus (NB.1.8.1) and Stratus (XFG) are currently the two main variants competing for global dominance, each with the potential to trigger a new wave of infection.
“In general, we are very cautious before assigning a new nickname. We then exchange proposals until we find a term that is recognizable enough to be useful in public communications,” explains Gregory.
“We started with mythological creatures (such as Orthrus and Kraken), then names of astronomical origin (Eris and Pirola), and this time we drew on meteorology, with cloud types such as Nimbus and Stratus.”
Frankenstein variants resulting from genetic recombination
NB.1.8.1 has a particularly complex family tree. It is a hybrid virus, resulting from a process called recombination: several viruses—all versions of the Omicron variant—infect the same cell and exchange genetic material.
NB.1.8.1 is a descendant of XDV, itself a hybrid of JN.1 and XDE—the latter being a cross between GW.5.1 and FL.13.4.
These recombinations have given “Nimbus” seven new mutations in its spike protein—the part of the virus that attaches to human cells—as well as 20 other mutations elsewhere in its genome.
XFG, meanwhile, is the result of a crossover between two previous variants: LF.7 and LP.8.1.2. It has four key mutations in its spike protein.
Nimbus adheres very well to our cells—and infects them effectively
Virological studies indicate that NB.1.8.1 binds strongly to the ACE2 receptor, which SARS-CoV-2 uses to enter human cells—more strongly than XFG and most other variants.
It also shows high infectivity in cells grown in the laboratory, suggesting that it is well equipped to spread. Its infectivity could be 2.5 times higher than that of LP.8.1, currently the most widespread variant in the world.
This could explain why it is gradually supplanting other variants — not by evading the immune system more effectively, but by infecting individuals more efficiently.
New COVID Variant : Stratus effectively evades the immune system
Two of the mutations present in the XFG spike protein are suspected of helping the virus evade antibodies that normally target certain areas of the protein. One of these mutations may even enable it to evade a broader class of antibodies, making it more difficult for the immune system to neutralize and facilitating the spread of the virus.
On the other hand, XFG binds less effectively to the human ACE2 receptor than some other variants. It may therefore be less effective at infecting cells or spreading on its own—unless it acquires new mutations.
New COVID Variant : Certain specific symptoms may need to be monitored.
Although the symptoms of these variants appear to be broadly similar to those of other versions of Omicron, British doctors have reported certain specific signs associated with NB.1.8.1: a “razor blade” sensation in the throat and gastrointestinal symptoms such as nausea, vomiting, diarrhea, heartburn, bloating, constipation, and abdominal pain.
However, these observations have not yet been confirmed by scientific studies.
In India, doctors have also noted hoarseness as a common symptom in patients recently seen in outpatient clinics at hospitals.
Nimbus is being closely monitored, but does not currently pose a major concern
The WHO has classified NB.1.8.1 as a variant under monitoring (VUM)—not a variant of interest (VOI) or a variant of concern (VOC). This means that it warrants close monitoring, but is not cause for alarm at this stage.
The overall risk to public health is currently considered “low.”
As for XFG, it is not (at this time) considered sufficiently concerning to be included on the WHO’s list of variants under monitoring. That said, like any emerging variant, it is subject to ongoing genomic surveillance, and all recorded viral sequences are closely analyzed to detect any potential changes, mutations, or health implications.
