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

Measles can now spread between states with documented outbreaks, making domestic travel a significant risk and potentially impacting your travel plans and public health response requirements.

A study published in NEJM Evidence details how measles, previously primarily imported via international travel, is now posing a significant risk through domestic travel. South Carolina experienced a large outbreak from October 2025 to April 2026, with 997 confirmed cases, at the time the largest since measles elimination in 2000. Over a two-week period in January, five separate importations of the virus from South Carolina to Washington state occurred. These introductions led to 26 confirmed cases in Washington residents. Whole genome sequencing was crucial in identifying these distinct importations, differentiating them from local transmission. Public health measures such as isolation, quarantine, and contact tracing were credited with containing the spread in Washington. The study highlights the increasing risk posed by domestic spread amidst declining vaccination rates.

The findings underscore a critical shift in how measles spreads within the US, moving from primarily international importations to significant domestic transmission chains. This necessitates a reevaluation of public health surveillance and response strategies, which may need to account for more frequent and widespread inter-state virus movement. The reliance on whole genome sequencing also points to its growing importance in understanding and managing infectious disease outbreaks in real-time.

This trend affects public health agencies, healthcare providers, and the general population by increasing the likelihood of encountering measles in non-traditional settings and potentially overwhelming local containment efforts. It also challenges narratives that downplay the current measles surge, emphasizing that domestic under-vaccination is a primary driver of US outbreaks, rather than solely international factors.