All about the 'Nimbus' and 'Stratus', the new COVID variants monitored closely by WHO

COVID-19 might not dominate the headlines anymore, but that doesn’t mean it has stopped evolving. In fact, behind the scenes, scientists are closely tracking several offshoots of the Omicron variant.

Two of those, unofficially nicknamed Nimbus and Stratus, are now on the radar for their quick global spread and potential to evade immunity. Here’s a closer look at what we know so far.

Nimbus (NB.1.8.1)

First spotted in early 2025, NB.1.8.1, or “Nimbus”, has made its presence felt quickly. It is currently listed as a ‘variant under monitoring’ (VUM) by the World Health Organization (WHO), - meaning they may require prioritised attention and tracking by health authorities due to their increased prevalence and potential public health implications. By late April, it had jumped from 2.5% to over 11% of all sequenced COVID-19 cases, especially across parts of Asia and the Western Pacific. Further, as per TODAY, 43% of cases in the U.S. are driven by this variant.

Nimbus is part of the Omicron family, and it's a recombinant virus — a mash-up of different viral lineages that combined genetic material. It descends from a complex mix involving XDV, which itself was formed by blending JN.1 and another hybrid known as XDE.

Symptoms of Nimbus

According to the Global Alliance for Vaccines and Immunizations (Gavi), the symptoms of the NB.1.8.1 variant include: runny nose, headache, fatigue, sneezing, and sore throat. Additionally, people also might feel a razor blade sensation in the throat, which is the defining symptom for this variant, and gastrointestinal symptoms, including: nausea, vomiting, diarrhea, heartburn, bloating, constipation, and stomach pain.

As of July 9, test positivity is at 3.1%, 0.1% higher than the previous week, according to the U.S. Centers for Disease Control and Prevention.The World Health Organization (WHO), in its May 23 evaluation, stated there’s no current evidence that Nimbus is more dangerous than the other Omicron subvariants circulating today. Further, it also states that the currently approved COVID-19 vaccines are still expected to be effective against this variant.

Stratus (XFG)

First detected in Canada, Stratus (XFG) it’s already made significant jumps into Europe and India.

By late May, it was behind a quarter of new cases in parts of Europe, overtaking even Nimbus. Stratus, too, has been classified by the WHO as ‘Variant under Monitoring”. Stratus is also a recombinant. It formed when two earlier variants, LF.7 and LP.8.1.2, crossed paths and exchanged genetic code. Like most of its Omicron cousins, it spreads quickly, prefers the upper respiratory tract, and seems less likely to lead to severe illness.

According to the current available data, symptoms of XFG are similar to those of other Omicron sub-variants which are typically mild upper respiratory symptoms.

Symptoms of the XFG

People infected with the XFG subvariant have reported a range of symptoms including: chills, shortness of breath, sore throat, fatigue, headache, nausea, vomiting, muscle and body aches, diarrhea, and in some cases, a loss of taste or smell.What sets XFG slightly apart, according to early reports, is one more noticeable symptom: hoarseness. Several individuals have described a rough or raspy voice, which may serve as a subtle clue when trying to distinguish this variant from other circulating strains.