A seasonal rise in covid-19 cases in Europe and North America has focused attention on a group of variants called XFG, which has dominated worldwide over the past year. A string of covid-19 variants has arisen since the virus emerged in 2019, but do we need to be concerned every time we hear of a new strain? Or have our past covid-19 infections and vaccinations given us a baseline level of protection?
Six years on from the height of the covid-19 pandemic, “we’re in this weird, uncharted territory”, says Danny Altmann at Imperial College London. “It’s not as good as we hoped, where we would just barely notice it, like the common cold. But it is not as bad as some of my colleagues feared, where we’re brewing the next disaster.”
In lab tests, XFG (sometimes known as Stratus) is better at evading antibody protection than previous covid-19 variants, which is thought to be why it has become dominant globally. But for most people, existing immunity from previous vaccinations and infections appears to be enough to protect against serious illness, leaving researchers counselling both calm and caution.
All covid-19 variants circulating today are descendants of Omicron, the last major strain to be named, which emerged in 2021. The virus “continues to evolve”, says Ian Mellis at Columbia University in New York, creating a succession of new variants that outcompete previous ones, causing waves of infection.
XFG was first detected in early 2025, and the World Health Organization started officially monitoring it that June. It now makes up around 60 per cent of sequenced covid-19 cases worldwide.
It emerged from an exchange of genetic material between two subvariants of an Omicron lineage called JN.1, says Mellis, perhaps when they both infected the same person. Earlier this year, Mellis and his colleagues reported the results of lab tests where they tested a range of covid-19 viruses, including XFG and another variant, nicknamed Nimbus, against antibodies found in human populations.
They found that although both variants bound to host cells slightly less well, this was more than offset by a greater ability to evade covid-19 antibodies, compared with past variants.
That ability to evade the immune response, on top of how widespread XFG is, may mean that more people will be susceptible to serious infection, says Altmann. “That may play out in a few more hospitalisations.” So far though, no evidence of increased disease severity, hospitalisations or deaths has been reported.
In the UK, for example, 1638 covid-19 cases were identified in hospital settings in the first week of October, a rise from 1328 cases the week before. But that is part of an expected seasonal rise, and lower than at the same time last year.
“We’re not seeing any evidence that [this year’s variants] are making people more ill or spreading faster than before,” says Alexander Allen at the UK Health and Security Agency, “and this year’s autumn [covid-19] vaccine has been specifically designed to protect them.”
Aris Katzourakis at the University of Oxford points out, however, that real-time assessment of the impacts of a virus can be very difficult. “It takes time for something that sweeps through to turn into hospitalisations and for those to be recorded.”
Earlier this year, Leena Azhar at Fakeeh College of Medical Sciences in Jeddah, Saudi Arabia, and her colleagues reviewed what we know about XFG, concluding that it exemplifies “quiet” evolution, more consistent with fine-tuning “than a shift toward greater virulence”.
Even though the virus is gradually drifting further from the original form, and many people in the UK haven’t been vaccinated for several years, “we’re making do with the immunity that we’ve got on board”, says Altmann. “That’s keeping a lid on it.”
That’s a very different situation compared with when Omicron emerged, he says. “It was so different, it seemed devastating.” But with the latest variants, “I’m not unduly excited or alarmed”, he says. “We’re still where we thought we were.”
This doesn’t mean a more dangerous strain couldn’t emerge in the future, for example, if many mutations accumulate in a single, immunosuppressed individual, which is how Omicron originated. “There is a risk that a new strain could emerge that causes a wave of infections again,” says Mellis.
There is also now far less monitoring that could provide early warning of such an event, he says. In particular, the amount of sequencing data has “fallen dramatically” over the last year.
Governments around the world also have worryingly “lackadaisical” attitudes to vaccine boosters, says Altmann. He points out that most children who are starting school have never been vaccinated, and it’s unclear what the consequences of that may be.