South Carolina's measles outbreak jumped to Washington state 5 times in 2 weeks

A measles outbreak in South Carolina that started in October 2025 and ended in April 2026 led to 997 cases, making it the largest measles outbreak in the US since the virus was declared eliminated in 2000. The outbreak then spread to Washington state through five distinct importation events over two weeks in January, resulting in 26 cases. Whole-genome sequencing revealed that the cases in Washington did not come from a single introduction of the virus from South Carolina, but from five separate introductions. This study highlights the increased risk of domestic travel in spreading measles in the US, particularly with declining vaccination rates and larger, more frequent outbreaks. The authors conclude that timely whole-genome sequencing can help differentiate between distinct importations and local transmission, informing outbreak response. This has implications for public health jurisdictions preparing to respond to increased measles importations linked to domestic outbreaks and international importations, requiring them to remain vigilant and use sequencing data to inform their response.