South Carolina measles outbreak reached Washington state five times in two weeks, study finds

Five importation events led to 26 Washington cases before officials stamped out the spread

By LineZotpaper AI staff

Published27 October 2026

Read Time2 min

Illustration: South Carolina measles outbreak reached Washington state five times in two weeks, study finds

Washington state health officials have detailed how South Carolina's measles outbreak reached their state through five separate importation events over two weeks in January, causing 26 infections among residents before the spread was halted. The study, published this week in NEJM Evidence, says the episodes signal that domestic travel now poses a greater risk than international travel for bringing measles back into US communities.

Measles has been declared eliminated in the United States since 2000, and the country's occasional cases have mainly come from international travelers, or unvaccinated US residents returning from abroad with the virus. But as vaccination rates have slipped nationwide in recent years, and outbreaks have grown bigger and more frequent, the new study highlights how domestic movement of infected people has become a growing concern.

The Washington cases were traced to South Carolina, where the measles 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 in the US since the virus was declared eliminated, though Pennsylvania's ongoing outbreak has since surpassed it.

In Washington, the five introductions led to 26 cases among residents before officials stamped out the spread, according to the study.

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Analysis

Why This Matters

South Carolina's outbreak, with nearly 1,000 confirmed cases, was the largest since measles was declared eliminated in the US in 2000, a measure of how far vaccination coverage has slipped.
The Washington infections show that domestic travel can now reintroduce the virus across state lines, not only arrivals from abroad.
The study gives state health officials evidence for how outbreaks spread within the country and where to focus surveillance and vaccination efforts.

Background

When measles was declared eliminated in the US in 2000, it meant the virus was no longer spreading continuously within the country. Cases that still occurred were typically traced to travelers arriving from countries where measles circulates, or unvaccinated US residents infected abroad. Measles is among the most contagious viruses, so a single importation can multiply quickly in undervaccinated communities. The Washington study suggests that as US outbreaks grow larger and more frequent, the internal movement of infected people now poses a comparable, or greater, risk.

What to Watch

The trajectory of Pennsylvania's ongoing outbreak, which has already surpassed South Carolina's as the largest recorded since elimination, and whether it seeds cases in other states.
Whether other states detect and contain importation events linked to active US outbreaks as quickly as Washington did.
Whether the cases prompt renewed attention to vaccination coverage in states where rates have slipped.

Sources

South Carolina's measles outbreak jumped to Washington state 5 times in 2 weeks · Ars Technica

Checked

9 claims in this story checked against 1 source: 9 supported.

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SupportedThe South Carolina outbreak reached Washington state through five importation events over two weeks.Ars Technica - All content: “detailing how an explosive outbreak in South Carolina reached their distant state through five distinct importation events over two weeks in January.”
SupportedThe five introductions led to 26 cases among Washington residents before officials stopped the spread.Ars Technica - All content: “In all, the introductions led to 26 cases among residents before officials stamped out the spread.”
SupportedThe study says domestic travel now poses a greater risk than international travel for bringing measles into US communities.Ars Technica - All content: “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.”
SupportedMeasles has been declared eliminated in the US since 2000.Ars Technica - All content: “Since measles was declared eliminated in the US in 2000”
SupportedOccasional US measles cases have mainly come from international travelers or unvaccinated US residents returning from abroad.Ars Technica - All content: “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.”
SupportedVaccination rates have slipped nationwide in recent years, while outbreaks have grown bigger and more frequent.Ars Technica - All content: “as vaccination rates have slipped nationwide in recent years and outbreaks grow bigger and more frequent”
SupportedThe South Carolina outbreak began on October 2, 2025, and was declared over on April 26, 2026.Ars Technica - All content: “South Carolina's measles outbreak began on October 2, 2025, and was declared over on April 26, 2026.”
SupportedOfficials confirmed 997 cases in South Carolina during the outbreak.Ars Technica - All content: “Officials confirmed 997 cases in the state during the outbreak.”
SupportedAt the time, it was the largest US measles outbreak recorded since measles was declared eliminated, but Pennsylvania's ongoing outbreak has since surpassed it.Ars Technica - All content: “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.”