To characterize the populations that selected the combination measles, mumps, rubella, and varicella vaccine (MMRV) as their first measles-containing vaccine and to inform interpretations of recent US vaccine policy changes.
Approach:
Study Design: Cross-sectional analysis of the Washington State Immunization Information System, including 213,445 children from King County, Washington, born between 2014 and 2021.
Data Evaluation: Evaluated the first measles-containing or varicella-containing vaccine administered and examined demographic characteristics, geographic distribution, clinic type, and Vaccines for Children (VFC) program eligibility.
Key Findings:
64% of children received MMR + VAR as their first vaccination, 18% received MMR alone, 15% received MMRV, and 4% received VAR alone.
Children receiving MMRV were more likely to be eligible for the VFC program and receive care at safety-net clinics.
95% of children completed both measles-containing and varicella-containing vaccine recommendations before age 4 years.
Interpretation:
Limitations:
Vaccine records may be incomplete for children vaccinated outside Washington State.
Findings from a single county may not be generalizable to other jurisdictions.
The study did not evaluate the effects of the September 2025 policy change.
