Five measles introductions detected in Washington state in two weeks — Ars Technica
Five separate measles introductions linked to an outbreak in South Carolina were recorded in Washington state over two weeks in January. They resulted in a total of 26 cases among state residents before the spread of the virus was brought under control.
As Ars Technica reports, the findings are based on a study by Washington state health officials published in NEJM Evidence. Epidemiological data and whole-genome sequencing showed that this was not one imported case followed by local transmission, but five separate introductions of the virus.
Disease clusters
The first case was detected in a state resident who returned from a trip to South Carolina and became ill on January 12 after contact with a person sick with measles. Genetic testing showed a close match between the virus and samples recorded in South Carolina. One close contact of this person also became ill and was subsequently isolated.
The largest cluster emerged after South Carolina residents visited public places in Washington state during their infectious period. Four people who had contact with them became infected. These individuals later attended schools, healthcare facilities and a church, and the total number of cases in this cluster reached 14 between January 15 and February 17.
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The role of isolation and contact tracing
Two more Washington residents became ill after returning from South Carolina on January 15–16, but no additional related cases were identified. In another episode, a person followed isolation and quarantine recommendations after returning from a trip, so no transmission occurred.
The fifth cluster included seven cases. A state resident became ill on January 19 after traveling outside the state and having contact with sick people, and visited a school and a healthcare facility during the infectious period. Six of the person's close contacts became ill. Viral genome sequences in five cases were nearly identical to samples from South Carolina.
The measles outbreak in South Carolina lasted from October 2, 2025, to April 26, 2026. During that time, 997 cases were confirmed in the state. According to the U.S. Centers for Disease Control and Prevention, 96% of the 4,588 confirmed measles cases this year are linked to an outbreak in the United States.
