Covid-19 levels are about the lowest they’ve ever been in the United States, but another new crop of virus variants once again threatens to disrupt the downward trend as the country heads into summer.

KP.2 — one of the so-called FLiRT variants — has overtaken JN.1 to become the dominant coronavirus variant in the United States, according to data from the US Centers for Disease Control and Prevention. Data through May 11 shows that it’s responsible for more than a quarter of cases in the country, which is nearly twice as many as JN.1. A related variant, KP.1.1, has caused about 7% of cases, CDC data shows.

FLiRT variants are offshoots of the JN.1 variant — all part of the broader Omicron family — that caused this winter’s wave. The acronym in the name refers to the locations of the amino acid mutations that the virus has picked up — some in places that help it evade the body’s immune response and others that help it become more transmissible.