A new, highly infectious COVID-19 variant called “Stratus” (scientifically XFG) is now dominant in the UK, raising concern among experts. According to the UK Health Security Agency (UKHSA), Stratus made up 40% of cases in mid-June, rising sharply from just 10% in May.

Stratus, a “recombinant” strain descended from Omicron, emerged when a person was infected with two Covid strains at once. Professor Lawrence Young said its subvariants, XFG and XFG.3, are “rapidly spreading” due to “new spike mutations” that help them evade the immune system. With fewer people getting booster shots and lower recent infection rates, immunity has waned, making more people vulnerable.

Though Stratus spreads fast, there’s “no evidence” it causes more severe illness. Vaccines are still believed to protect against serious outcomes. The World Health Organization recently labeled it a “variant under monitoring,” citing its global growth—now responsible for 22% of worldwide cases.

Another strain, Nimbus, is also on the rise, going from 2% of UK cases in April to 17% in June. It’s been linked to a “razor blade” sore throat and swollen glands, but overall COVID cases are down. Still, experts stress the risk remains, especially for the vulnerable.