A Covid variant that has become dominant in the US is spreading rapidly in Britain, with cases and hospital admissions rising sharply as the country heads into the winter respiratory illness season.
The strain, known as XFG or Stratus and colloquially dubbed “American Covid”, accounted for more than 60 per cent of sequenced Covid-positive samples in the UK between August 17 and August 30, according to official figures.
The rise comes as the overall burden of Covid on the UK’s health system has increased significantly. Some 4,589 cases were recorded in September, up from 1,374 in August — an increase of roughly 230 per cent.
Hospital admissions have risen even more sharply, from 950 in August to 4,939 in September.
The proportion of hospital tests returning positive for Covid also increased from 5.9 per cent to 7 per cent in the seven days to September 30, according to figures from the UK Health Security Agency.
The figures are likely to attract close attention from policymakers as the NHS enters a period in which pressure from respiratory illnesses traditionally increases. Yet health officials have sought to draw a distinction between rising transmission and evidence of a more dangerous virus.
Alex Allen, consultant epidemiologist at the UKHSA, said: “The strains currently going around, including those known as XFG, are closely related to strains that have been with us for over a year, so this isn’t something new.”
XFG was first identified in southeast Asia in 2025 before becoming the dominant strain in the US. It is a hybrid sub-variant of Omicron, the family of coronavirus variants that drove successive waves of infections during the pandemic.
Mutations are thought to have increased its transmissibility, potentially contributing to the recent rise in cases alongside seasonal factors. There is, however, no evidence supplied here that XFG causes more severe illness than previous strains.
Symptoms can include fever, cough and sore throat, with hoarseness of the voice reported as a distinctive feature.
The winter Covid vaccination programme for older and vulnerable people has been designed to provide protection against XFG and related variants, according to reports.
The health service has stressed that the symptoms can resemble other common respiratory illnesses.
“The symptoms are very similar to symptoms of other illnesses, such as colds and flu,” the NHS says.
“Most people feel better within a few weeks, but it can take longer to recover. For some people, it can be a more serious illness and their symptoms can last longer.”
For the economy, the immediate concern is less the return of the pandemic-era restrictions than the potential impact of a sustained rise in sickness and hospitalisation on an already stretched workforce and health service.
The NHS advises people with symptoms who have a high temperature or feel too unwell to work or attend school to “try to stay at home and avoid contact with other people”.
“You can go back to your normal activities when you feel better or do not have a high temperature,” it adds.
The rise in XFG cases therefore presents a familiar but materially different challenge: a virus that is no longer an emergency on the scale seen in 2020, but which can still impose significant costs when infections translate into hospital demand and lost working time.
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