Use of Nowcasting to Estimate Real-Time Transmission Trends During a Measles Outbreak - South Carolina, October 2025-March 2026.

During October 2025-March 2026, South Carolina experienced the largest measles outbreak in the United States in approximately 30 years, with 997 cases reported. The South Carolina Department of Public Health reported confirmed measles cases to CDC through regular updates of provisional (initially reported) line-list data, including essential date fields for each case (rash onset and public health report dates). As in all disease surveillance systems, because provisional data are subject to reporting delays, determining current transmission trends is challenging. CDC developed a nowcasting estimation model to generate real-time estimates that correct for reporting delays. Real-time estimates were generated for the final case counts and time-varying effective reproduction number (Rt), an indicator of whether the rate of new infections is increasing or decreasing. Whether case reporting during an active measles outbreak would be stable enough to support accurate real-time nowcasting was unknown before the data in this report were analyzed. During this outbreak, nowcasts improved estimates of cumulative outbreak size compared with provisional data, and Rt estimates generally captured underlying transmission trends. Nowcast estimates of daily cases were generally accurate, although accuracy declined in January when provisional data were less complete than expected because of a rapid increase in cases. These results demonstrate that if essential date fields are consistently collected, nowcasting can support situational awareness and decisions about response capacity by providing real-time estimates of daily case counts and transmission trends.