XFG (Stratus) Covid variant drives US wave as wastewater signals surge

A new Covid-19 variant, XFG — also known as Stratus — is driving a nationwide surge in infections, according to federal and local health data. Wastewater monitoring shows high or higher Covid activity across the United States for the week of Sept. 7, with the latest figures indicating 29 states reporting high or very high activity as of Sept. 16. Officials have said the variant’s rapid rise means it now accounts for the majority of infections in the country, even as hospitalizations remain relatively low compared with earlier waves.

The XFG variant is a hybrid of two Omicron lineages, LF.7 and LP.8.1.2, and is believed to be more contagious due to mutations in its spike protein that help it bind to cells and evade antibodies from prior infection or vaccination. Health officials say the variant’s symptom profile resembles those of earlier Omicron strains, with fever, chills, cough, shortness of breath, sore throat, congestion, runny nose, fatigue, body aches, headache, nausea, vomiting and diarrhea reported among patients. The emergence of Stratus has coincided with broader debates over Covid vaccination policy and messaging, even as public health officials emphasize keeping up to date with vaccines and boosters where appropriate.

By Sept. 9, CDC data indicated XFG accounted for about 80 percent of US Covid infections, up from roughly 3 percent in May. The variant was first detected in January in Southeast Asia and was identified in the United States two months later. The rapid shift to dominance is occurring as the agency notes its state-level estimates of variants come with low confidence because states submit data inconsistently; nevertheless, wastewater data — derived from hundreds of sites across the country — are interpreted as a real-time signal of transmission.

The nationwide rise in XFG comes as overall indicators show some softening in the spread. Across the week ending Sept. 13, hospitalizations attributed to Covid stood at about 1.5 per 100,000 people, down from 2.5 per 100,000 the week before. Test positivity declined to about 9.5 percent for that same period, and emergency department visits related to Covid fell from 1.6 percent to 1.2 percent in the latest week reported. These metrics suggest the current wave, while persistent, may be moderating on a national level even as localized surges occur.

Nine states are reporting growing infections, with a mix of wastewater activity levels across the country. North Dakota data were not available, but South Dakota currently shows very high wastewater activity, while Minnesota and Delaware report high activity. The remaining states — including Wisconsin, Michigan, New York, Vermont, and New Jersey — show moderate to high levels or trends toward growth. Officials note that newer infections in these states appear to be driven by XFG, rather than older strains, though the quality and granularity of variant data at the state level varies.

Several factors may influence the geographic pattern of spread. In South Dakota, the concentration of Native American reservations and disparities in access to health care have historically affected outbreak dynamics. In Minnesota, colder winter months drive more indoor gatherings, potentially elevating transmission risk. Delaware’s aging population, with a sizable share of residents aged 60 and older, may also shape vulnerability to infection.

In parallel with the viral shift, policy debates continue over vaccination strategy. A U.S. CDC vaccine advisory panel voted against a universal recommendation for vaccines, endorsing instead a framework of individual decision-making about vaccination. Public health officials say vaccines and boosters remain a key tool for reducing severe disease, even as the circulating variant shows higher transmissibility.

The CDC stresses that wastewater is an indirect measure of community transmission and should be interpreted alongside clinical data. While the current signal points to ongoing transmission, experts caution that data gaps at the state level can limit precision in estimating how quickly the variant is spreading. Health officials urge continued testing, especially for those with symptoms or exposure, and to seek medical care if symptoms worsen. Individuals at higher risk due to age or underlying conditions should consult their health care provider about vaccination and boosters as new guidance and options become available.

Overall, the trajectory of XFG’s spread remains a moving target, with national indicators showing a slowing trend but persistent regional variation. As the variant becomes the dominant circulating strain, health systems will continue to monitor its impact on hospitalizations, case severity and population immunity, while health authorities maintain messaging on vaccination, testing and mitigation measures to reduce transmission.