Domestic Travel Carries Measles Across the Country: Five Importations in Two Weeks
An explosive measles outbreak in South Carolina reached Washington state through five distinct importation events within two weeks in January, seeding 26 cases among Washington residents before officials stamped out the spread. The account comes from a study by Washington state health officials published this week in NEJM Evidence, and it carries a warning for every other state: measles in America is now a domestic-travel problem, not just an international one.
South Carolina's outbreak began on October 2, 2025, and was declared over on April 26, 2026, with 997 confirmed cases statewide. At the time it was the largest measles outbreak recorded since the virus was declared eliminated in the United States; Pennsylvania's still-growing outbreak has since surpassed it.
On January 12, 2026, a Washington resident developed a rash after exposure to ill contacts during travel in South Carolina; the case was laboratory-confirmed as measles. One secondary case in a close contact was confirmed by epidemiologic linkage, with rash onset on January 31. Both people adhered to isolation and quarantine, and no further cases followed. The viral whole genome from the index case matched South Carolina sequences with 100 percent nucleotide identity across 12,089 to 15,677 base pairs.
The second importation did the most damage. South Carolina visitors moved through public places in Washington while infectious, and four close contacts fell ill. While contagious, those four visited schools, healthcare facilities, and a church. Nine of their contacts plus one person exposed in a public setting then fell ill, producing a 14-case cluster spanning January 15 to February 17.
The third importation involved two state residents who returned from South Carolina and fell ill on January 15 and 16; no additional cases were linked to them. A fourth importation involved a resident returning from travel who complied with isolation and quarantine recommendations, again with no onward cases. In the fifth event, a resident who had contact with ill contacts during out-of-state travel fell ill on January 19 and visited a school and a healthcare facility while contagious; six close contacts fell ill, for a seven-case cluster from January 19 to January 29. Genome sequences from five of those cases were nearly identical to ones seen in South Carolina.
Investigators generated high-quality whole viral genomes for 10 of 11 laboratory-confirmed cases: 10 confirmed by polymerase chain reaction testing at the Washington State Public Health Laboratories and one by commercial serology. Genomic analyses supported the epidemiologic conclusion that the five importations were independent of one another rather than one hidden chain. The authors credit adherence to isolation and quarantine with likely limiting onward transmission in undervaccinated communities.
National figures from the Centers for Disease Control and Prevention frame the stakes. As of October 22, 2026, the United States had 4,588 confirmed measles cases this year across 43 outbreaks, with 96 percent of cases outbreak-associated. Only 18 cases were reported among international visitors; 4,404 of the total were linked to a domestic outbreak. Meanwhile Pennsylvania's outbreak has topped 1,300 cases, the country's largest in three decades, after beginning in late April with no sign of slowing.
The backdrop is eroding immunity. Nationwide, 92.4 percent of kindergartners received the MMR vaccine in the 2025-2026 school year, down from 95.2 percent in 2019-2020; in South Carolina the figure was 92.1 percent in 2023-2024, down from 95 percent. Two doses are 97 percent effective at preventing infection, and about 95 percent coverage is considered necessary to block transmission. Nearly two-thirds of South Carolina's 997 cases involved children and teens ages 5 to 17.
The implication is that every state is now every other state's importation risk. Washington's experience shows that rapid epidemiology, whole-genome sequencing, and cooperative isolation can stop the spread at 26 cases. But with Pennsylvania past 1,300 cases and still climbing, each new pocket of low coverage makes the next import-and-contain cycle more likely. In November, the Pan American Health Organization reviews US elimination status, and the verdict will render judgment on a year of 4,588 cases.