Delayed or Absent First Dose of Measles, Mumps, and Rubella Vaccination.

Importance: A total of 1723 measles cases have been reported as of November 12, 2025, in the US, reaching their highest levels since elimination in 2000. MMR (measles, mumps, and rubella) vaccination coverage has decreased, and factors associated with delayed and missed vaccination since the COVID-19 pandemic are not well explored. Objectives: To characterize coverage and trends of timely MMR vaccination and assess factors associated with late vaccination and nonvaccination by 2 years of age. Design, Setting, and Participants: In this cohort study, infants who accessed routine care within the first 2 months, first year, and second year of life were followed up for 24 months to assess vaccination outcomes between January 1, 2018, and April 30, 2025. Participants were children seeking care within Truveta Data, an electronic health record database from a collective of US health care systems. Exposures: Timely receipt of routine 2- and 4-month immunizations and adherence to the American Academy of Pediatrics well child visit schedule. Main Outcomes and Measures: The primary outcome was timely, late, or no receipt of MMR by 2 years of age. Associations with primary exposures and sociodemographic factors were modeled using mixed-effect logistic regression with state-level random effects. Models were stratified by pre- vs post-COVID-19 MMR eligibility, with results after the COVID-19 pandemic reported as primary. Results: In this study of 321 743 children (166 017 boys [51.6%]) with regular access to care, 78.4% (252 250 of 321 743) received their first MMR vaccination on time, increasing from 75.6% (12 840 of 16 978) in 2018 to 79.9% (39 739 of 49 767) in 2021, then decreasing to 76.9% (40 306 of 52 388) in 2024. The strongest factors associated with no MMR vaccination by 2 years was late administration of a child's 2-month vaccines (adjusted odds ratio [AOR], 6.96 [95% CI, 6.60-7.34]) and 4-month vaccines (AOR, 6.16 [95% CI, 5.84-6.50]). Conclusions and Relevance: In this cohort study of children with regular access to care, most received their MMR vaccine on time, but the proportion not receiving the MMR vaccine by 2 years of age has increased since the COVID-19 pandemic. Children who did not receive their 2- and 4-month vaccines on time were significantly more likely to not receive any MMR vaccine by 2 years, highlighting opportunities for intervention.