California’s summer COVID surge has proved to be particularly strong and enduring, surprising experts with its tenacity as it storms into a third month.

The strength of this summer’s COVID surge probably is largely related to the ever-more infectious subvariants that continue to emerge as the coronavirus evolves, said Dr. Elizabeth Hudson, regional chief of infectious disease at Kaiser Permanente Southern California. A dizzying number of related subvariants — collectively dubbed FLiRT — have emerged in recent months. One in particular, KP.3.1.1, has been picking up steam at a startling pace and has become the most common strain nationwide.