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... In a study published this week in NEJM Evidence, Washington state health officials highlight that risk, detailing how an explosive outbreak in South Carolina reached their distant state through five distinct importation events over two weeks in January. In all, the introductions led to 26 cases among residents before officials stamped out the spread.

South Carolina's measles outbreak began on October 2, 2025, and was declared over on April 26, 2026. Officials confirmed 997 cases in the state during the outbreak. At the time, it was the largest measles outbreak recorded since the virus was declared eliminated, but Pennsylvania's ongoing outbreak has since surpassed it.

The study in Washington relied heavily on epidemiological data, linking cases among contacts and exposure sites. But also key to the work was whole-genome sequencing, which made clear that cases didn't just stem from one single introduction of the virus from South Carolina that then spread among Washington residents; instead, it revealed five separate introductions.

"As public health jurisdictions prepare to respond to increased measles importations linked to domestic measles outbreaks, while remaining vigilant for international importations, timely whole-genome sequencing can supplement epidemiologic information to help differentiate distinct importations from local transmission and inform outbreak response," the authors conclude. ...