Children who received the combination measles, mumps, rubella, and varicella vaccine as their first measles-containing vaccine were more likely to receive care through safety-net clinics and participate in the Vaccines for Children program than children who received other vaccine schedules. The findings characterize the populations that historically selected the combination vaccine and may help inform interpretation of recent US vaccine policy changes. An accompanying invited commentary discussed the potential equity implications of the findings.
Researchers conducted a cross-sectional analysis of the Washington State Immunization Information System, including 213,445 children from King County, Washington, born between 2014 and 2021 who received at least one measles-containing or varicella-containing vaccine between 2015 and 2025. They 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 associated with vaccine selection.
The primary outcome was the first vaccine received: coadministered measles, mumps, and rubella vaccine plus varicella vaccine (MMR + VAR), the combination measles, mumps, rubella, and varicella vaccine (MMRV), measles, mumps, and rubella (MMR) vaccine alone, or varicella (VAR) vaccine alone. Researchers also characterized the populations that selected MMRV as their initial vaccine.
Overall, 64% of children received MMR + VAR as their first vaccination, 18% received MMR vaccine alone, 15% received MMRV, and 4% received VAR vaccine alone. MMRV use remained stable throughout the study period. Compared with children who did not receive MMRV, those who did were more likely to receive catch-up vaccination between ages 16 and 47 months, identify as Hispanic, American Indian or Alaska Native, Black, Native Hawaiian or Pacific Islander, multiracial, or another or an unspecified race, reside in southern King County, be eligible for the VFC program, and receive vaccination at safety-net clinics.
Among all children, 95% completed both measles-containing and varicella-containing vaccine recommendations before age 4 years. Among children who initially received MMR vaccine alone, 78% later received a varicella-containing vaccine before age 4 years, whereas 69% of those who initially received VAR vaccine alone subsequently received a measles-containing vaccine.
The researchers noted that combination vaccines may reduce the number of injections, clinic visits, and costs for families. They wrote that reduced access to MMRV in safety-net settings could disproportionately affect VFC-eligible children by reducing vaccine options and increasing barriers to timely series completion.
In an accompanying invited commentary, Elizabeth L. Cope, PhD, of Academy Health, and colleague wrote that the study provides important context for the September 2025 Advisory Committee on Immunization Practices recommendation to remove MMRV as an option for children younger than 4 years and to exclude it from VFC coverage. They noted that the study identifies the populations most likely to be affected by reduced access to the combination vaccine and emphasized the importance of considering equity when evaluating vaccine policy.
Researchers acknowledged several limitations. Vaccine records may have been incomplete for children vaccinated outside Washington State, and findings from a single county in a universal-purchase vaccine state may not be generalizable to other jurisdictions. In addition, the study described historical vaccine-use patterns and did not evaluate the effects of the September 2025 policy change.
“This population might be at risk of not receiving recommended vaccines if options become limited,” wrote lead study author Meaghan S. Fagalde, MPH, of Public Health–Seattle & King County, and colleagues.
Disclosures: Dr Chow reported receiving travel support from the American Academy of Pediatrics to attend its National Conference in 2023, 2024, and 2025. No other disclosures were reported.
Source: JAMA Network Open