Washington Traces Five Separate Measles Importations to South Carolina Outbreak
Whole-genome sequencing linked 26 measles cases in Washington to five distinct importations from a large South Carolina outbreak, highlighting the growing role of domestic travel in virus spread.
A study published in NEJM Evidence details how an explosive measles outbreak in South Carolina led to five separate importation events in Washington state over a two-week period in January 2025. The South Carolina outbreak, which began on Oct. 2, 2025 and ended on Apr. 26, 2026, produced 997 confirmed cases, making it the largest in the United States since measles was declared eliminated in 2000.
Washington health officials used epidemiological investigations and whole-genome sequencing to differentiate the importations. The first case involved a resident who fell ill on Jan. 12 after returning from South Carolina; genome analysis matched the virus to South Carolina strains, and only one close contact was infected. A second importation resulted in a cluster of 14 cases linked to four residents who had been exposed to South Carolina visitors and subsequently visited schools, health facilities and a church. Two additional importations each involved a returning traveler who isolated without further spread. The fifth importation, identified on Jan. 19, led to a cluster of seven cases after the infected traveler visited a school and a health facility, with six close contacts becoming ill.
"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 wrote.
Officials credited quarantine, isolation and contact-tracing efforts with limiting further transmission. CDC data show that of the 4,588 measles cases reported in the United States this year, only 18 were linked to international travelers, while 96 % were associated with domestic outbreaks, underscoring the shift from international to domestic travel as the primary risk factor.
The report also notes that statements by anti-vaccine activist Robert F. Kennedy Jr., who claimed the United States is managing the outbreak better than any other country, are contradicted by the data, which indicate that the surge is driven largely by under-vaccinated communities within the United States.