Measles was declared eliminated in the United States in 2000, and the cases that appeared afterward came mostly from international visitors or from unvaccinated US residents returning from abroad. As vaccination rates have slipped across the country and outbreaks have become larger and more frequent, travel within the US now carries a greater risk than imported cases from other nations.
Washington state health officials describe that risk in a study published this week in NEJM Evidence. They detail how a major outbreak in South Carolina reached their distant state through five distinct importation events over two weeks in January. Those introductions led to 26 cases among Washington residents before officials brought the spread under control.
The South Carolina outbreak began on October 2, 2025, and was declared over on April 26, 2026. Officials confirmed 997 cases in the state. At the time, it was the largest measles outbreak recorded since the virus was declared eliminated, though an ongoing outbreak in Pennsylvania has since surpassed it.
The Washington analysis drew heavily on epidemiological data, which allowed investigators to link cases among contacts and exposure sites. Whole genome sequencing was also central to the work. It showed that the cases did not come from a single introduction of the virus that then spread among Washington residents. Instead, the virus arrived separately five times.
The first importation involved a resident who developed the characteristic rash on January 12 after returning from a trip to South Carolina, where they had been exposed to a person with measles. Genome sequencing found the virus closely matched sequences from South Carolina cases. One close contact of that person also became ill and was isolated. No other cases were linked to this importation.
The authors say the findings point to a need for preparation. "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," they conclude.
