Fast-spreading COVID Nimbus and Stratus hit Spain

Spain is experiencing a gradual increase in COVID-19 cases as two new Omicron sub-variants, Nimbus (NB.1.8.1) and Stratus (XFG), begin to circulate more widely.

While public health experts emphasise that most infections remain mild, especially among vaccinated individuals, authorities are monitoring the situation closely to prevent potential local outbreaks.

What Are Nimbus and Stratus?

Nimbus (NB.1.8.1)

Nimbus has been designated a Variant Under Monitoring (VUM) by the World Health Organisation (WHO). Identified in January 2025, it has spread rapidly across Asia and Europe, accounting for around 11% of sequenced cases in some regions by late April. Experts describe Nimbus as a highly transmissible variant due to its ability to bind strongly to human ACE2 receptors, which may help explain its rapid spread.

The nickname “Nimbus” refers to a type of cloud rather than pop culture references. Researchers chose it to distinguish the variant as a significant lineage in the global COVID-19 landscape. According to Prof. T. Ryan Gregory, part of the team tracking these variants, Nimbus has “established itself as a new lineage evolving among hosts, capable of causing waves in some regions.”

Stratus (XFG)

Stratus is spreading rapidly in Europe, India, and North America, though it is not currently listed as a WHO-monitored variant. Unlike Nimbus, Stratus shows a stronger ability to evade antibodies, allowing it to infect individuals who may have partial immunity from vaccination or prior infection. Researchers note that while Stratus may not bind as efficiently to human cells as Nimbus, its immune evasion capabilities give it a competitive edge in certain populations.

Genetic Background

Both variants are products of genetic recombination, where multiple Omicron lineages infect the same cell and exchange genetic material. Nimbus carries seven new spike protein mutations and additional changes elsewhere, increasing its ability to infect cells. Stratus has spike mutations that may help it evade antibodies, potentially increasing the likelihood of reinfections.

Symptoms of Nimbus and Stratus

Nimbus (NB.1.8.1) Symptoms

• Severe sore throat (“razor blade” sensation)
• Fatigue
• Cough
• Runny nose
• Headache
• Mild fever or chills
• Hoarseness
• Gastrointestinal issues such as nausea, vomiting, or diarrhoea

Stratus (XFG) Symptoms

• Persistent dry cough
• Sore or scratchy throat
• Hoarseness
• Fever or chills
• Fatigue
• Headache
• Nasal congestion or runny nose
• Nausea, vomiting, or diarrhoea

While symptoms overlap, Nimbus appears to cause more pronounced throat irritation and gastrointestinal effects, whereas Stratus may present more as a persistent dry cough. Most infections are mild, but early detection, isolation, and treatment are crucial to prevent complications and slow transmission.

Public Health Response

Surveillance and Testing

Spanish health authorities are increasing genomic sequencing and monitoring hospital admissions to track these variants. Laboratories are analysing samples to detect changes in symptoms or transmissibility. Hospitals have been urged to report unusual symptom patterns promptly to regional health authorities.

Vaccination and Prevention

COVID-19 vaccines remain effective, particularly updated boosters, which help protect against severe illness, hospitalisation, and death. Dr. Alberto Martínez Martínez, a specialist in Cadiz, advises, “Vaccination continues to be the best defence. People should stay current with boosters and seek medical attention if symptoms develop, especially older adults or those with underlying conditions.”

Guidance for Expats and Visitors

For those living in or visiting Spain, authorities recommend monitoring for symptoms, avoiding close contact with anyone showing signs of illness, practising good hand hygiene, and wearing masks in crowded indoor spaces. Expats should also ensure they have access to healthcare providers or medical insurance and know where to get tested if necessary.

Implications for Spain

Although neither Nimbus nor Stratus currently poses a high public health risk, their rapid spread underscores that COVID-19 is still evolving. Authorities stress that vigilance, vaccination, and adherence to protective measures remain essential. Public health messaging is aimed at both residents and visitors to reduce confusion and reinforce preventive measures.

Spain’s experience highlights the need for ongoing surveillance and public awareness. By understanding the differences between Nimbus and Stratus, recognising symptoms, and staying vaccinated, expats and tourists can help protect themselves and the broader community.