Domestic measles transmission rises as a primary source of US outbreaks

A report published in NEJM Evidence documents how an extensive measles outbreak in South Carolina, which totaled 997 cases between October 2025 and April 2026, spread to Washington state. Health officials identified five separate instances in January 2026 where infected travelers moved the virus from South Carolina to Washington, resulting in 26 secondary infections in the latter.

Historically, measles cases in the United States were primarily linked to international travel following the virus's elimination status in 2000. However, public health experts note that declining vaccination rates across the country have made domestic movement a significant vector for the disease. The recent data suggests a shift in how the virus traverses the nation.

While the South Carolina incident was the largest recorded since 2000 at the time of its conclusion, subsequent events, including an ongoing outbreak in Pennsylvania, have surpassed that scale. Health officials continue to monitor how decreasing immunization coverage impacts the frequency and reach of these domestic transmissions.