State to state
South Carolina’s measles outbreak jumped to Washington state 5 times in 2 weeks
International travel used to be the main risk of measles cases in the US—not anymore.
Since measles was declared eliminated in the US in 2000, the country’s occasional cases have come mainly from international travelers—visitors from far-flung places where the virus still spreads freely — or unvaccinated US residents who return from abroad with a highly infectious souvenir. But as vaccination rates have slipped nationwide in recent years and outbreaks grow bigger and more frequent, domestic travel now poses a greater risk than international incursions.
In a study published this week in NEJM Evidence, Washington state health officials highlight that risk, detailing how an explosive outbreak in South Carolina reached their distant state through five distinct importation events over two weeks in January. In all, the introductions led to 26 cases among residents before officials stamped out the spread.
South Carolina’s measles outbreak began on October 2, 2025, and was declared over on April 26, 2026. Officials confirmed 997 cases in the state during the outbreak. At the time, it was the largest measles outbreak recorded since the virus was declared eliminated, but Pennsylvania’s ongoing outbreak has since surpassed it.
The study in Washington relied heavily on epidemiological data, linking cases among contacts and exposure sites. But also key to the work was whole genome sequencing, which made clear that cases didn’t just stem from one single introduction of the virus from South Carolina that then spread among Washington residents; instead, it revealed five separate introductions.
“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 conclude.
Importations
In their analysis, health officials identified the first importation in a resident who fell ill with the telltale rash on January 12 after returning from a trip to South Carolina, where they were exposed to a person sick with measles. Genome sequencing found that the virus closely matched sequences from South Carolina cases. One close contact of the Washington case fell ill and was isolated. No other cases were linked to this importation.
In a second importation event, South Carolina residents visited public places while infectious. As a result, four residents with close contact with the South Carolina visitors fell ill. While infectious, the four residents visited schools, healthcare facilities, and a church. Nine of their contacts and one person exposed in a public location fell ill, leading to a total cluster of 14 cases that spanned January 15 to February 17.
In the third importation event, two state residents returned from South Carolina and fell ill on January 15 to 16. No additional cases were linked to these residents. Similarly, a fourth importation event occurred when a resident returned from travel. The person adhered to isolation and quarantine recommendations, and no cases were linked to this one.
In the fifth importation event, a Washington state resident returned from out-of-state travel where they had contact with ill contacts and fell ill themselves on January 19. While contagious, they visited a school and a healthcare facility. Six close contacts fell ill. In all, this cluster of seven cases spanned January 19 to January 29. The genome sequences from five of the cases were nearly identical to those seen in South Carolina.
Officials credit basic public health work of quarantines, isolation, and contact tracing with keeping the virus from spreading farther than it did.
Contradicting Kennedy
While the study highlights how readily this virus can spread among states amid declining vaccination, it also contradicts a common talking point from Health Secretary Robert F. Kennedy Jr. Kennedy, a prominent anti-vaccine advocate, has downplayed the current measles surge in the US and aggressively defended against suggestions that his decades of anti-vaccine misinformation and disinformation have contributed to the surge in cases and drop in vaccination.
Instead, Kennedy claims that there is a “global outbreak” and the US is “doing a better job than any country in the world at controlling the measles outbreak.” Both claims are false. Dozens of other countries have lower case rates than the US. While some countries, including neighboring Canada and Mexico, are also seeing large increases in cases, the soaring case tally in the US is not being driven by international imports, but by domestic spread in under-vaccinated communities.
According to data from the Centers for Disease Control and Prevention, only 18 of the country’s 4,588 confirmed measles cases this year were reported among international visitors. Of all the cases, 4,404 (96 percent) are linked to a domestic outbreak. The US has had 43 new outbreaks this year.
