A new COVID-19 variant, known as XFG or colloquially as “Stratus,” is spreading in the United States and around the globe, health officials report.
The WHO has described XFG as a recombinant of two earlier COVID-19 strains: F.7 and LP.8.1.2.
While LP.8.1.2 is itself among the leading variants in the United States, making up 31 percent of cases, XFG has distinguished itself by accumulating nine additional mutations in the spike protein compared with the still-dominant NB.1.8.1, which remains the top strain at 43 percent.
The variant was nonexistent in case counts through March, reached 2 percent in April, rose to 6 percent by late May, and 11 percent by early June, before climbing to the most recent 14 percent.
Globally, the WHO included XFG on its variant watchlist after the uptick in reported cases in June. Data gathered from 38 countries found XFG represented just 7.4 percent of positive tests in the first week of May, but that figure jumped to 22.7 percent by month’s end. The United Kingdom is among the hardest-hit outside Southeast Asia, with Stratus accounting for 30 percent of positive COVID-19 tests.
The WHO report found that samples of XFG submitted to the international genetic database leaped from 7 percent in May 2025 to nearly 23 percent a few weeks later.
WHO has evaluated XFG’s overall public health risk as “low” at a global level, stating in the report that “currently approved COVID-19 vaccines are expected to remain effective to this variant against symptomatic and severe disease”.
Constantly Changing
The CDC uses a national genomic surveillance system to monitor new and existing variants, utilizing virus samples collected by the National SARS-CoV-2 Strain Surveillance (NS3) program and sequences from academic, commercial, and public health laboratories.
According to the CDC, “SARS-CoV-2, the virus that causes COVID-19, is constantly changing and accumulating mutations… New variants of SARS-CoV-2 are expected to continue to emerge. Some variants will emerge and disappear, while others will emerge and continue to spread and may replace previous variants”.
To provide up-to-date data, the CDC issues both weighted estimates, based on laboratory-confirmed results, and Nowcast estimates, which model variant proportions for the most recent two-week intervals.
These Nowcast estimates help forecast the prevalence of emerging variants before full sequencing data is available, though projections remain less certain when a variant is in the early stages of spreading. Updated CDC variant data is released biweekly.
