Highlights
- The Centers for Disease Control and Prevention (CDC) developed a nowcasting model to support response during a 2025-2026 measles outbreak in South Carolina.
- Nowcasting can add value to measles outbreak response when surveillance systems provide timely, high-quality, and stable reporting.
Background
During active emergency response outbreaks, case counts can be incomplete because of delays in seeking healthcare, laboratory testing, reporting, and case investigation. These delays can make an outbreak appear to be declining when transmission is still increasing.
From October 2025 through March 2026, South Carolina experienced a large measles outbreak with nearly 1,000 reported cases. To help address reporting challenges in this outbreak, the South Carolina Department of Public Health (SCDPH) shared regular provisional case data with CDC, including both rash onset dates and public health report dates. CDC developed a nowcasting pipeline that adjusted for reporting delays and estimated current case counts and the time-varying reproductive number, or Rt. Rt estimates inform whether an outbreak is increasing, stable, or decreasing.
The value of timely reporting and modeling during an outbreak
CDC implemented a real-time nowcasting pipeline using the EpiNow2 modeling framework to estimate daily measles incidence after adjusting for reporting delays. The pipeline generated automated reports within minutes of receiving updated line lists, allowing CDC and SCDPH to rapidly distinguish true changes in transmission. In December 2025, for example, provisional case data indicated declining trends while nowcasts estimated the trend was likely increasing. In part due to these estimates, DPH maintained consistently high levels of staffing and reporting over the holiday period and began hiring additional case investigators and contact tracers.
During periods of stable reporting, nowcasts closely matched final case counts and accurately characterized transmission. During a period of rapid case growth in early January, reporting was less complete than expected, which weakened model performance. Nevertheless, nowcasts still improved estimates of outbreak size compared with provisional case counts alone. These real-time estimates complemented routine surveillance by providing reporting-adjusted measures of incidence and transmission, supporting timely situational awareness throughout the outbreak. Learn more about methods and view published results in an accompanying Morbidity and Mortality Weekly Report (MMWR).
Future work
Nowcasting can support outbreak response. In South Carolina, collaboration between CFA, SCDPH, and the CDC Career Epidemiology Field Officer program (CEFO) fostered rapid adoption of nowcasting which helped public health officials interpret outbreak trends, distinguish real declines from reporting delays, and make decisions about response staffing.
The collaboration involved high-quality case investigation at the state and local level, with SCDPH collecting key date fields and sharing timely data – and technical expertise in data science at CDC. Other health departments interested in applying this work and partnering with CFA should contact us here.
Acknowledgements
- CFA aims to use data, modeling, and analytics to respond to outbreaks in real time to inform effective decision-making.
- CEFOs strengthen state, tribal, local, and territorial epidemiology capacity for public health preparedness and response.
- SCDPH works to protect, promote, and improve the health and well-being of everyone in South Carolina.
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