What We Know About South Carolina's Measles Spread to Washington

Washington state health officials traced five separate measles introductions from South Carolina's outbreak into their state over a two-week span in January, according to a study published this week in NEJM Evidence and reported by Ars Technica. The finding, built on whole-genome sequencing rather than epidemiological tracing alone, illustrates a shift researchers say is reshaping measles risk in the United States: domestic travel, not just arrivals from abroad, is now driving outbreaks.

How did measles travel from South Carolina to Washington?

Since the United States declared measles eliminated in 2000, the virus's occasional appearances have come mostly from international travelers or unvaccinated residents returning from trips abroad, according to the study cited by Ars Technica. But as vaccination rates have slipped nationally and outbreaks have grown larger and more frequent, officials in Washington found that South Carolina's outbreak reached their state through domestic travel alone — not a single importation followed by local spread, but five distinct introductions identified through genome sequencing.

The first case surfaced on January 12, when a Washington resident developed the telltale measles rash after a trip to South Carolina, where the person had been exposed to someone infectious, per the study. Sequencing confirmed the virus closely matched South Carolina case samples.

How many measles cases resulted from the Washington importations?

Across the five importation events, the introductions led to 26 cases among Washington residents before officials contained the spread, the study found. The clusters varied widely in size. The first importation produced just one additional case — a close contact who was isolated, with no further spread documented.

A second importation proved far more consequential. South Carolina residents visited public places in Washington while infectious, exposing four Washington residents who later visited schools, healthcare facilities, and a church while sick themselves. Nine of their contacts and one person exposed in a public location also fell ill, producing a cluster of 14 cases spanning January 15 to February 17, according to the study.

A third importation involved two Washington residents who had traveled to South Carolina and developed symptoms January 15 to 16; the study material reviewed did not report additional spread tied to that cluster.

"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 study authors concluded, as quoted by Ars Technica.

What does the timeline of introductions show?

• January 12: First Washington case identified; traveler returns from South Carolina with measles symptoms after exposure there.
• January 15–16: Two additional Washington residents fall ill after separate South Carolina travel, marking a third importation.
• January 15–February 17: A fourth cluster, seeded by infectious South Carolina visitors to Washington, grows to 14 cases spanning more than a month.
• Across roughly two weeks in January: Genome sequencing ultimately distinguishes five separate viral introductions rather than one outbreak that spread locally.

Without whole-genome sequencing, officials might have assumed a single chain of local transmission. Instead, the genetic data showed each cluster traced back independently to South Carolina, according to the study.

How big was South Carolina's outbreak, and how does it compare nationally?

South Carolina's outbreak began October 2, 2025, and was declared over April 26, 2026, with 997 confirmed cases in the state, the study and Ars Technica report. At the time it ended, it was the largest measles outbreak recorded since the US eliminated the disease in 2000 — a record since surpassed by an ongoing outbreak in Pennsylvania, according to the reporting.

Why does domestic travel now pose a growing measles risk?

The Washington study frames its findings as a warning for other states: as unvaccinated pockets grow and outbreaks like South Carolina's expand, travelers moving between US states — not just international arrivals — can seed new clusters far from the original outbreak. The authors argue that distinguishing domestic importations from local transmission chains requires genomic tools, not contact tracing alone, so health departments can calibrate their response and vaccination outreach accordingly, per the study summarized by Ars Technica.

The Washington cases were ultimately stamped out, according to the reporting, but the pattern — five separate seedings from a single distant outbreak within two weeks — underscores how quickly a large domestic outbreak can cross state lines when vaccination coverage gaps persist.