Scientists are warning of a new Covid variant that's "just getting started" and now spread to 20 countries.
XEC was first detected in Germany in June, and has since been reported in the UK, Denmark and Ukraine.
Experts have said that the variant could be the "next big challenge" for hospitals.
Dr Eric Topol, director of the Scripps Research Translational Institute in California, said on X (formerly Twitter): "At this juncture, the XEC variant appears to be the most likely one to get legs next."
He said it may take "many weeks, a couple of months, before it really takes hold and starts to cause a wave".
He added: "XEC is definitely taking charge. That does appear to be the next variant. But it's months off from getting into high levels."
Scientists believe the new variant is unlikely to be as deadly as previous ones, but could be more contagious.
Professor Francois Balloux, director of the Genetics Institute at University College London, told BBC News that although XEC has a "slight transmission advantage" over other recent Covid variants, vaccines should still offer good protection.
Symptoms include a high temperature, aches, tiredness, a cough or sore throat, trouble sleeping and a runny nose.
Those at risk of catching Covid are urged to get a booster vaccine, still available through the NHS.
The countries with cases of XEC are:
UK
France
Germany
Denmark
Belgium
Netherlands
Poland
Norway
Luxembourg
Slovenia
Taiwan
Italy
Ireland
Ukraine
Spain
Israel
Portugal
China
USA
Canada
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