South Carolina’s measles outbreak exploded into one of the worst in the U.S., with state health officials confirming 99 new cases in the past three days.

The outbreak centered in Spartanburg County grew to 310 cases over the holidays, and spawned cases in North Carolina and Ohio among families who traveled to the outbreak area in the northwestern part of the state.

State health officials acknowledged the spike in cases had been expected following holiday gatherings. A growing number of public exposures and low vaccination rates in the area are driving the surge, they said. As of Friday, 200 people were in quarantine and nine in isolation, state health department data shows.

“The number of those in quarantine does not reflect the number actually exposed,” said Dr. Linda Bell, who leads the state health department’s outbreak response. “An increasing number of public exposure sites are being identified with likely hundreds more people exposed who are not aware they should be in quarantine if they are not immune to measles.”

— The Associated Press