Inferring Outbreak Size and Onset Date for a Multistate Measles Outbreak Using Whole Genome Sequencing — Arizona and Utah, 2025
- Presentation Day/Time: Tuesday, April 21, 2:55 PM
- Presenter: Christine Wang, DVM, PhD, MPH, Pima County Health Department and Arizona Department of Health Services
The Issue
- On August 8, 2025, the Mohave County Department of Public Health reported the first Arizona-Utah measles outbreak case with rash onset on August 6. By December 31, Mohave County and southwest Utah reported 335 cases. Ascertaining true outbreak size and onset date is challenging because of community reluctance to participate in investigations or seek healthcare.
What We Did
- To evaluate evidence for undetected cases, we estimated the size and onset date of the outbreak using phylogenetic analysis of measles whole genome sequencing (WGS) data. Phylogenetic analysis can estimate attributes about measles viruses in this outbreak, which we use to understand outbreak size and onset date.
What We Found
- Phylogenetic analysis suggests the outbreak might have started 1.5 months or more before initial detection, and the estimated outbreak size was at least 6.5 times the reported size. Thus, undetected cases were possible both during the outbreak and before the first reported case.
What This Means
- Phylogenetic analysis with whole genome sequences enables estimation of epidemiologic parameters that are challenging to obtain using classic investigation methodologies.
