Following the nationwide decline in vaccination rates and the growth of domestic outbreaks, domestic travel now presents a higher risk of spreading measles than international visitors or returning unvaccinated residents. Since measles was declared eliminated in the United States in 2000, occasional cases typically originated from international travelers or returning residents.
Outbreak Details and Genomic Sequencing
In a study published in NEJM Evidence, Washington state health officials detailed how an explosive outbreak in South Carolina reached Washington through five separate importation events during a two-week period in January. These introductions resulted in 26 resident cases before authorities successfully contained the spread.
The South Carolina outbreak started on October 2, 2025, and concluded on April 26, 2026, with a confirmed total of 997 cases. Although it initially stood as the largest measles outbreak since the virus was deemed eliminated, an ongoing outbreak in Pennsylvania has since exceeded it in size.
Washington researchers utilized epidemiological data connecting contact cases and exposure sites alongside whole-genome sequencing. This advanced sequencing confirmed that the infections did not stem from a single virus introduction from South Carolina, but rather from five distinct events.
The study’s authors concluded that as public health agencies prepare for rising measles importations tied to domestic outbreaks while watching for international cases, timely whole-genome sequencing can aid epidemiological data by distinguishing separate importations from local transmission to guide responses.
First Identified Importation Event
Health officials pinpointed the initial importation in a resident who developed a rash on January 12 following a trip to South Carolina, where the individual encountered an infected person. Genome sequencing demonstrated that the virus matched sequences taken from South Carolina cases.
One close contact of the patient in Washington subsequently contracted the illness and was placed in isolation. Authorities confirmed that no additional cases were connected to this specific introduction.
