Genome Sequencing Reveals Five Measles Importations from South Carolina to Washington

Measles has resurged in the US as vaccination rates declined, with South Carolina experiencing an outbreak from October 2025 to April 2026 that confirmed 997 cases—the largest since the virus was declared eliminated in 2000. The South Carolina outbreak demonstrated a shift in epidemiological risk: domestic travel now poses greater risk than international incursions for measles spread. The virus reached Washington state through five separate importation events in January, resulting in 26 cases among residents before containment.

Researchers at Washington state health officials documented these five introductions in a NEJM Evidence study, using whole genome sequencing to distinguish them as distinct viral introductions rather than stemming from a single import that spread locally. The first confirmed case involved a resident who fell ill January 12 after exposure during a trip to South Carolina; genome sequencing matched this virus to South Carolina cases. One close contact of this case fell ill and was isolated with no further transmission tied to that importation.

The study concludes that as vaccination coverage declines and outbreaks grow larger and more frequent, timely whole genome sequencing can supplement epidemiological data to differentiate new importations from local transmission, informing more effective public health response.