Measles Keeps Hopping From South Carolina to Washington State

For years, measles cases in the United States were traced mostly to travelers who caught the virus overseas and brought it home. That pattern is changing. An outbreak centered in South Carolina has reached Washington state five separate times in just two weeks, and the spread is happening between states rather than across international borders.

The disease is among the most contagious known to medicine. It moves through the air when an infected person coughs, sneezes or simply breathes, and the virus can remain in a room for up to two hours after that person leaves. People can pass it on for roughly four days before a rash appears, so someone may be spreading measles without realizing they are sick. Each jump between states gives the virus a fresh set of households, schools and workplaces to reach.

Public health officials rely on vaccination as the main defense. Two doses of the MMR vaccine are about 97 percent effective at preventing measles, and community protection depends on enough people being immune to break chains of transmission. When immunization rates drop in a region, a single traveler, school exposure or family gathering can start a cluster that spreads well beyond its origin.

The shift also complicates how states respond. Tracing close contacts, isolating exposed people and checking immunization records are slower when the chain runs through domestic travel that gets none of the entry screening that international arrivals face. Health departments in each affected state are likely to focus on those steps and on encouraging vaccination, especially for children and anyone who has not confirmed two doses.