Nimbus and Stratus: Cloudy With a Chance of Covid

As we venture into the end of summer, there’s no denying that COVID-19 is making its move again. Covid waves always follow a seasonal pattern - two waves per year: winter and summer or fall. So far, the smallest (not in terms of deadliness though, just the number of infected) wave hit during the winter of 2020-2021, while the Omicron surge of 2021-2022 took the crown for the biggest wave. Last year was a bit of a wild card, with the summer/fall wave being slightly taller than the winter one. However, so far, this year hasn’t had much of a summer/fall wave (yet) but hold on tight because things are looking a little more intense as we approach the winter.

From December 2024 to February 2025, we experienced what could be called "Wave X," a quieter episode compared to earlier waves, but it still made its presence felt. Fast forward to August 2025, and we’re beginning to see the rise of the next wave—a little like what we saw in 2023 and 2024. According to Mike Hoerger’s post on August 8, 2025, the early signs suggest this wave might not reach the explosive heights of the summer wave two years ago. Instead, it’s expected to be closer in size to the Delta wave from four years ago (2021-2022). But don’t get too comfortable just yet!

At the beginning of 2025, the most prevalent variant globally was XEC, followed by KP.3.1.1. However, XEC’s circulation began to decline in February while another strain, LP.8.1, became more common in March. Since mid-April, the presence of LP.8.1 has slightly decreased, with NB.1.8.1 emerging as one of the most detected variants.

So, the two main contenders for dominance in this wave? Nimbus (NB.1.8.1) and Stratus (XFG). Both named by T. Ryan Gregory's team in early 2025. Yes, the names are a bit "weather-forecast-y," and yes, we can’t help but think of them as rival clouds duking it out for the spotlight. 

Nimbus (NB.1.8.1): The "razor blade throat" variant designated as VUM (Variant Under Monitoring) by the World Health Organization (WHO) in May 2025. It was first detected in January 2025 and has made quick progress, especially in Asia, particularly China and Hong Kong, where it caused early surges, before spreading globally. By March 2025, it was detected in the U.S. and spread to at least 22 countries, including Australia, Thailand, the UK, and several U.S. states (e.g., California, New York). In Northern Ireland it was linked to stomach-related issues, such as nausea, vomiting, diarrhea constipation, bloating, abdominal pain and even heartburn. Nimbus' mutations that increase its ability to bind to the ACE2 receptor and evade the immune system, contributing to its rapid spread. Despite its growth, it is not yet considered a Variant of Concern (VOC). It was first detected in January 2025 and quickly gained traction, particularly in Asia, before spreading globally. Its rate of 120% per week in regions like Hong Kong suggested that Nimbus could eventually outpace other variants like LP.8.1 and its parent XFC if it follows a similar trajectory in Western countries. And it did. But a stronger variant emerged. 

Stratus is now emerging as a strong competitor. And wastewater data support XFG dominance too. The CDC’s variant proportion estimates may contain errors due to the small number of sequences reported, a limitation the CDC acknowledges when presenting the current data. The bad news is that the CDC is receiving almost no data from New York State. Ongoing state dashboard updates may be obscuring high transmission rates in densely populated areas around NYC.

Stratus, a recombinant variant of LF.7 and LP.8.1.2, with four spike protein mutations (His445Arg, Asn487Asp, Gln493Glu, Thr572Ile) was first collected on January 27, 2025, with likely origins in Southeast Asia, particularly India, where it reached over 50% of cases by mid-June. By June 22, 2025, it was detected in 38 countries, comprising 22.7% of global sequences (up from 7.4% a month earlier). High prevalence in India (50%+), Spain (42%), the UK (30%), and the U.S. (30%). In Australia, it was 5% of sequences by late June. It was officially designated a Variant Under Monitoring (VUM) by the World Health Organization (WHO) on June 25, 2025. Despite being relatively new to the scene, Stratus has already shown promising signs of dominance in regions like Southeast Asia and the UK, where it accounts for a significant portion of cases. By June 2025, Stratus was responsible for 21% of U.S. cases, with more subvariants (XFG.3: 14.9%, XFG.2 : 6.7%) seen in early Aug, and its spread continues to accelerate.

With its enhanced immune evasion and growing presence globally, Stratus is a variant to watch closely as we head into the colder months of 2025. The battle between Nimbus and Stratus is far from over, and it might ultimately shape the next wave of the pandemic.



REFERENCES

https://www.easterneye.biz/new-covid-variant-confirmed-uk/

https://pmc19.com/data/

https://x.com/jpweiland

https://x.com/yunlong_cao/status/1932035033436749905

https://x.com/michael_hoerger

https://x.com/RajlabN

https://public.tableau.com/app/profile/raj.rajnarayanan/viz/ConvergentLineages-VariantSoup-World/G20

https://www.gavi.org/vaccineswork/eight-things-you-need-know-about-new-nimbus-and-stratus-covid-variants

https://www.gavi.org/fr/vaccineswork/huit-choses-savoir-nouveaux-variants-covid-19-nimbus-stratus

Guo C, Yu Y, Liu J, Jian F, Yang S, Song W, Yu L, Shao F, Cao Y. Antigenic and virological characteristics of SARS-CoV-2 variants BA.3.2, XFG, and NB.1.8.1. Lancet Infect Dis. 2025 Jul;25(7):e374-e377. doi: 10.1016/S1473-3099(25)00308-1. Epub 2025 Jun 5. PMID: 40484018.

Mellis IA, Wu M, Hong H, Tzang CC, Bowen A, Wang Q, Gherasim C, Pierce VM, Shah JG, Purpura LJ, Yin MT. Antibody evasion and receptor binding of SARS-CoV-2 LP. 8.1. 1, NB. 1.8. 1, XFG, and related subvariants. bioRxiv. 2025:2025-07.

Yi B. Evaluation of the evolution of SARS-CoV-2 Omicron variant and the spreading of LP. 8.1 and NB. 1.8. 1. medRxiv. 2025:2025-06.

Gupta SD. The Resurgence of COVID-19: Are We Going to Face Yet Another Pandemic?. Journal of Health Management. 2025 Jun;27(3):307-.

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