Since measles was declared eliminated in the US in 2000, the country’s occasional cases have come mainly from international travelers—visitors from far-flung places where the virus still spreads freely — or unvaccinated US residents who return from abroad with a highly infectious souvenir. But as vaccination rates have slipped nationwide in recent years and outbreaks grow bigger and more frequent, domestic travel now poses a greater risk than international incursions.
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.
In their analysis, health officials identified the first importation in a resident who fell ill with the telltale rash on January 12 after returning from a trip to South Carolina, where they were exposed to a person sick with measles. Genome sequencing found that the virus closely matched sequences from South Carolina cases. One close contact of the Washington case fell ill and was isolated. No other cases were linked to this importation.
