Emergence of New COVID-19 Variants: Nimbus and Stratus Under Review

As the COVID-19 pandemic continues to evolve, the World Health Organization (WHO) has identified two new sub-variants, NB.1.8.1, informally dubbed 'Nimbus', and XFG, known as 'Stratus'. Since their identification, these variants have rapidly spread across multiple continents, raising questions about their implications for public health and vaccination campaigns.

The WHO currently lists these variants among the six ‘variants under monitoring’, a classification that indicates their potential risk to global health systems. According to WHO’s Technical Advisory Group on Virus Evolution, the available evidence on NB.1.8.1 does not suggest additional public health risks compared to other circulating Omicron-descendant lineages. As of late April 2025, Nimbus accounted for 11% of sequenced cases, a significant rise from 2.5% just one month earlier. In contrast, Stratus has shown even more rapid dissemination, particularly in Europe, where it constituted 25% of cases by late May 2025.

Professor T. Ryan Gregory of the University of Guelph, who is part of the research group responsible for naming these variants, emphasized that the choice of the name 'Nimbus' reflects its unique genetic makeup and rising prominence in the viral landscape, rather than any pop culture references. 'Nimbus and Stratus are the two main variants competing for dominance globally, and either one has the potential to kick off a surge,' said Professor Gregory.

Both variants have been classified as 'Frankenstein variants', due to their origins of genetic recombination from other Omicron variants. NB.1.8.1 has acquired seven new mutations in its spike protein, critical for viral entry into human cells, while XFG has four key mutations. Laboratory studies suggest that Nimbus binds more effectively to the ACE2 receptor, which SARS-CoV-2 utilizes to enter cells, potentially explaining its rapid spread.

Despite the alarming spread, experts assert that current COVID-19 vaccines are still likely to be effective against these new variants. Laboratory tests indicate a reduced antibody effectiveness against both variants, with XFG showing a two-fold drop and NB.1.8.1 a 1.6-fold drop in neutralization. However, vaccinated individuals, particularly those with updated boosters, retain protection against severe disease, hospitalization, and death.

In terms of symptoms, while they broadly resemble those of previous Omicron variants, there are indications that patients infected with NB.1.8.1 may experience a distinctive sensation in the throat, along with gastrointestinal symptoms. Indian doctors have noted hoarseness as a common symptom among patients presenting with this variant.

At present, WHO has designated NB.1.8.1 as a Variant Under Monitoring, indicating it is noteworthy but not alarming. The overall risk to public health remains assessed as low. XFG, while spreading quickly, is not currently under enhanced WHO scrutiny. Both variants will continue to be monitored through genomic surveillance to track their evolution and any potential public health impacts.

As the pandemic landscape shifts, health authorities remain vigilant, balancing the need for public awareness with the necessity of maintaining calm amid ongoing research and data collection regarding these emerging variants.