DENVER (AP) — Colorado became the third U.S. state to detect a case of the omicron variant in a woman who had recently traveled to Africa, state health officials announced Thursday.

The Colorado case comes after two other confirmed omicron variant cases were found in California and Minnesota.

Officials in Colorado detected the new case in a woman who is a resident of Arapahoe County, an area just east of Denver. She recently traveled to Africa as a tourist, the state health department said in a statement.

“The first thing that’s required for a virus to mutate is that it has to be out in a community. The longer a covid is out in a community… it will continue to mutate,” said Dr. Reggie Washington, chief medical officer at Presbyterian/St. Luke’s Medical Center and Rocky Mountain Hospital for Children.

The woman who is infected with the new variant is fully vaccinated, but had not received her booster shot yet, health officials said.

“The World Health Organization notified us last week that there was a new variant that was behaving a little more vigorously than the previous variants that they were observing. So what that means is that this particular variant appears to be easily passed on from person to person, and that’s the frightening thing,” Washington said.

She is experiencing mild symptoms, health officials said. The woman’s close contacts in Colorado have tested negative, they said.