Experts Raise Alarm Over ‘Stratus’ – The Most Contagious COVID Variant Yet Dominating the UK
A new and highly transmissible COVID-19 variant, nicknamed “Stratus” and officially labeled XFG, is rapidly becoming the dominant strain in the UK—prompting warnings from health experts about a possible new wave of infections.
Stratus is believed to have mutations that help it evade the immune system more effectively than previous strains, making it significantly more infectious.
According to the UK Health Security Agency (UKHSA), Stratus is now the most prevalent COVID strain in England. In just three weeks, its share of cases surged from about 10% in May to nearly 40% by mid-June.
A new hybrid COVID-19 variant known as “Stratus” is sweeping across the UK, raising alarms among health experts. Dubbed a “Frankenstein strain” due to its recombinant origins, Stratus—officially labeled XFG—is a direct descendant of the highly infectious Omicron variant.
Stratus emerged as a hybrid when an individual was infected with two strains of COVID simultaneously, giving rise to a new, more contagious version. According to the UK Health Security Agency (UKHSA), the strain has surged to become the most dominant in England, accounting for nearly 40% of cases by mid-June—up from just 10% in May.
Virologist Professor Lawrence Young of Warwick University told MailOnline that both the original XFG strain and its offshoot, XFG.3, are “rapidly spreading.” He attributes their rapid rise to “new spike mutations which make these variants more able to evade the immune response.”
“With fewer people getting booster jabs this spring, and infection levels having dipped in recent months, population immunity is waning,” Professor Young warned. “This leaves more people vulnerable to infection with XFG and XFG.3, potentially fueling a fresh wave. But it’s still unclear how large that wave could be.”
Crucially, there is currently no evidence that Stratus causes more severe illness than previous strains. Vaccination, Professor Young stressed, remains highly effective in preventing hospitalisation and severe outcomes.
Just a week before Stratus’s surge, the World Health Organization (WHO) designated it a “variant under monitoring,” citing its rapid spread across multiple countries. Though the global risk is still considered “low,” the WHO reported that Stratus already makes up 22% of infections worldwide and appears to have a “significant growth advantage” over other variants.
Another newcomer, the “Nimbus” variant, is also making waves. It rose from just 2% of cases in April to 17% in June, according to UKHSA data.
Still, total COVID case numbers remain relatively low. In the week ending June 29, only 5.4% of UKHSA-tested samples returned positive results—down slightly from 7% the week prior, which marked the highest rate of 2025 so far.
While neither Stratus nor Nimbus appear to bring new symptoms, doctors caution that Nimbus may cause a particularly painful “razor blade” sore throat. Dr. Michael Gregory, Regional Medical Director for NHS England in the North West, confirmed the variant is spreading swiftly and often presents with “swollen neck glands” as well.
Health experts emphasize that even mild COVID infections can still pose serious risks—particularly for older adults and those with weakened immune systems.