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 travel and family gatherings during the school break. 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.”

Since the outbreak started in October, Bell has warned that the virus was spreading undetected in the area. Hundreds of school children have been quarantined from school, some more than once.

South Carolina is one of two active hot spots for measles. The other outbreak is on the Arizona-Utah border, where 337 people have gotten measles since August.

Last year was the nation’s worst year for measles spread since 1991, end-of-year data from the Centers for Disease Control and Prevention shows. The U.S. confirmed 2,144 cases across 44 states.

And as the one-year anniversary of the Texas-New Mexico-Oklahoma outbreak approaches — which sickened at least 900 people and killed three — health experts say the vaccine-preventable virus is on the verge of making a lasting comeback in the U.S.

At that point, the U.S. would lose its status of having eliminated local spread of the virus, as Canada did in November. International health experts say the same strain of measles is spreading across the Americas.

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