(AP) — Stanford University researchers say a cluster of children in California have developed a rare, polio-like syndrome within the past year that quickly paralyzed one or more of the children’s arms or legs.

Stanford’s Dr. Keith Van Haren says about 20 cases have been identified in California in the past year and a half and they’re the only ones in the U.S.

Van Haren says the five cases he studied had been vaccinated against polio. He plans to talk to the media about the illness Monday evening from Lucille Packard Children’s Hospital at Stanford.

Dr. Jane Seward of the Centers for Disease Control and Prevention cautions that the research is still underway in California, and there are a variety of infectious diseases that can cause childhood paralysis.