Health authorities in the United States, United Kingdom, and other countries are closely watching trends in COVID-19 infections as new variants spread. Two emerging sublineages—NB.1.8.1 (commonly nicknamed “Nimbus”) and XFB/XFG (often called “Stratus”)—have been associated with a noticeable uptick in case numbers as indoor gatherings increase during colder weather and people spend more time inside. Wastewater surveillance and testing programs indicate that transmission is occurring beyond what official case counts capture, similar to earlier waves of the pandemic, even though overall surveillance (e.g., widespread testing) has declined compared with peak years.
Both Nimbus and Stratus are descendants of the Omicron family of SARS-CoV-2 variants. NB.1.8.1 is an Omicron-derived lineage identified in early 2025 and designated by the World Health Organization (WHO) as a “variant under monitoring.” It has increased in prevalence globally, detected in more than 20 countries, and appears to have mutations that may enhance transmissibility, contributing to its rising share among circulating strains.
Stratus (XFB/XFG) is a recombinant Omicron variant, with genetic material shared between two earlier subvariants. WHO similarly lists it as a “variant under monitoring” due to patterns of spread and potential immune-evasion changes. Both variants thus continue the broader pattern of SARS-CoV-2 evolution toward greater transmissibility rather than increased severity.
Clinical reports and health expert observations suggest that many infections with these variants cause symptoms similar to other recent COVID-19 lineages, with fever, cough, fatigue, headaches, and congestion commonly reported. Several clinicians and county health officials have noted intensely sore throat sensations, sometimes described in media reports as feeling like “razor blades” in the throat, along with hoarseness and voice changes—though these symptoms are part of a broader range and not universally experienced.
For NB.1.8.1 specifically, symptoms documented by health agencies include typical respiratory and systemic signs: sore throat, fever, runny nose, cough, fatigue, muscle aches, and occasionally nausea or gastrointestinal discomfort. The variation in symptom intensity reflects differences in immune status, prior infection history, and vaccination coverage.
Despite these variants’ prominence in case trends, current evidence does not indicate they cause more severe disease than earlier Omicron strains. Hospitalization and death rates remain much lower than during the early pandemic thanks to widespread immunity from vaccines and prior infections. WHO and national health agencies continue to categorize these lineages as variants under monitoring, reflecting careful observation without declaring them variants of concern. The primary issue is increased transmissibility, which can still strain healthcare systems if many infections occur in a short period, especially among older adults or people with preexisting conditions.
Health experts emphasize that vaccination—especially updated seasonal COVID-19 vaccines and boosters—remains strongly recommended to reduce the risk of severe illness, hospitalization, and death. Current vaccines targeting recent strains are expected to retain effectiveness against severe outcomes from Nimbus and Stratus infections, even if they do not completely prevent infection or transmission. Precautions such as testing when symptomatic, staying home while ill, improving ventilation, masking in crowded indoor spaces, and maintaining good hygiene are recommended to limit spread and protect vulnerable populations.
The emergence and spread of Nimbus and Stratus underscore that COVID-19 continues to evolve and circulate globally, even if it has receded from the emergency phase seen earlier in the pandemic. Public health guidance stresses steady vigilance over alarm, focusing on personal protection and community safety rather than fear. The virus’s adaptability means new subvariants are likely to continue appearing, but current data show that existing vaccines and preventive strategies are still valuable tools. As countries monitor case trends and wastewater signals, health authorities advocate informed, responsible responses, recognizing that COVID-19 remains part of the respiratory virus landscape
