NB 1.8.1 and XFG sub-variants spread fast but aren’t more severe; Who is monitoring new developments?

Two newly named COVID-19 sub-variants – NB.1.8.1 and XFG, nicknamed “Nimbus” and “Stratus” – are fuelling a fresh rise in infections across the globe. Though neither variant appears to cause more severe illness, scientists are closely monitoring their spread, evolution, and potential impact on vaccine effectiveness.

Here’s what you need to know about these variants now making headlines:

1. Fast Spread, Not Fiercer Symptoms
Discovered in early 2025, NB.1.8.1 has grown rapidly, making up 11% of sequenced cases globally by April—up from just 2.5% a month earlier. XFG has spread even faster, now accounting for 25% of European cases. Both variants have led to rises in infections and hospitalisations in regions like Asia and India. Still, neither shows signs of causing more severe illness compared to other Omicron variants, according to the World Health Organization (WHO).

2. The Names ‘Nimbus’ and ‘Stratus’ Aren’t Just for Fun
The unofficial names come from a network of scientists and variant trackers led by Prof T. Ryan Gregory. “Nimbus” is the first new nickname since “Pirola” in 2023, while “Stratus” was coined due to its competition with NB.1.8.1. Naming helps with public communication, especially as both variants spread. The cloud-themed names reflect this round of variant evolution.

3. Both Are Recombinant ‘Frankenstein’ Viruses
NB.1.8.1 is a genetic mash-up formed from multiple Omicron lineages through recombination. It has seven new mutations on its spike protein and twenty more elsewhere, enhancing its infectivity. XFG is also a hybrid – combining LF.7 and LP.8.1.2 – and has four notable spike protein mutations.

4. Nimbus Is Better at Infecting Cells
Lab tests show NB.1.8.1 binds more tightly to the ACE2 receptor than most variants, including XFG. This gives it an edge in spreading, not because it dodges immunity more effectively, but because it infects efficiently. Its infectivity is estimated at 2.5 times higher than LP.8.1, currently the dominant strain.

5. Stratus Evades Immunity More Efficiently
Two of XFG’s spike mutations help it escape antibody detection. One of these affects a broad antibody class, potentially making the virus harder to neutralise. However, its ability to latch onto human cells is weaker than Nimbus, meaning it may need further mutations to truly compete.

6. Vaccines Still Offer Strong Protection
Although lab tests show a small drop in antibody response—around 1.6-fold for NB.1.8.1 and 2-fold for XFG—vaccines, especially updated boosters, still provide robust protection. Vaccinated individuals are expected to have strong defence against severe illness, hospitalisation, and death.

7. Watch for Possible New Symptoms
Both variants produce symptoms typical of Omicron, but doctors in the UK and India report some distinctive signs: a razor-like throat pain, gastrointestinal issues (including nausea, bloating, and diarrhoea), and hoarseness. These have yet to be confirmed in large-scale studies but may offer early warning signs.

8. Monitored, Not Feared – Yet
Nimbus is now listed by WHO as a Variant Under Monitoring (VUM), not as a Variant of Interest or Concern. Its current public health risk is considered “low.” Stratus, though not officially monitored by WHO, is also under scrutiny via routine genomic surveillance, especially as it rapidly spreads in India and Europe.

For now, these variants signal a reminder that COVID-19 hasn’t vanished. While no cause for alarm, health authorities stress the importance of continued vaccinations, vigilance, and tracking as the virus continues to evolve in unexpected ways.