Children late on 2- and 4-month vaccines less likely to receive MMR by 2 years of age

Children who did not receive their 2- and 4-month routine and recommended vaccines were more likely not to have any measles, mumps, and rubella (MMR) vaccine doses at 2 years of age, according to updated data since the COVID-19 pandemic. The data, published on January 2, 2026, in JAMA Network Open, stress the importance of on-time adherence to the pediatric immunization schedule according to the study authors.¹

How does routine vaccination impact MMR coverage at 2 years?

The investigative team aimed to determine what factors associated with late or no MMR vaccination have changed since the COVID-19 pandemic, following up with over 321,000 infants for MMR vaccination by 2 years of age. The authors noted increasing vaccine hesitancy since the pandemic, and the impact on other routine and recomended vaccines.

"Understanding factors associated with untimely MMR vaccination is important to aid pediatricians in having early conversations with parents about vaccinations, helping to allay hesitancy and ensure children are on track," wrote the investigators. "We explored vaccine coverage and factors associated with timely and delayed childhood immunization among children younger than 2 years in the US."

The study featured 321,743 children (51.6% male) who had regular access to care, with the primary outcome being timely, late, or no receipt of MMR by 2 years of age. According to the authors, "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."

The cohort study used an EHR database that featured over 120 million individuals from a collective of health care systems across the United States, with data collected in May of 2025. Individuals were born from 2017 to 2023. Vaccines in the primary childhood series included MMR, Hib, DTaP, PCV, IPV, and rotavirus. These were included if they were received between birth and 2 years of age, and were classified as appropriately administered, and either early, on time, or late, based on Centers for Disease Control and Prevention (CDC)-recommended age and dosing intervals (with a 4-day grace period before the minimum age of administration).^1,2

No immunizations at 2, 4 months led to less MMR coverage

Results revealed that 78.4% of children in the cohort received their first MMR vaccine on time, increasing from 75.6% in 2018 to 79.9% in 2021. Coverage then decreased to 76.9% in 2024, with the strongest factors associated with no MMR at 2 years being late administration of the child's vaccines at 2 months of age (adjusted odds ratio [AOR], 6.96 [95% CI, 6.60-7.34]) and 4 months of age (AOR, 6.16 [95% CI, 5.84-6.50]).

"The post-2021 decrease in timely MMR vaccination corresponded to an increase in the proportion of children with no MMR vaccine recorded by 2 years, which increased from 5.3% (2617 of 49 760) in 2020 to 7.7% (4044 of 52 388) in 2024," wrote the study authors. Across the study period, late MMR vaccination remained consistent, at approximately 14%. Early vaccination was rare:

• Less than or equal to 1% (0.6%) in 2018
• 0.9% in 2024
• Increase to 2.9% for children with a 12-to-21-month visit in 2020.

Additionally, early vaccination represented approximately 6.3% of doses for all 2-month vaccines and 2.2% for all 4-month vaccines. Overall, DTaP coverage rates were highest while rotavirus were lowest, with consistent findings across the study period.

"Vaccine hesitancy is a complex issue, with multifactorial influences, including personal beliefs, access to health care, and additional sources of hesitancy and delays due to the COVID-19 pandemic," the authors wrote. "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," they concluded.

References:

1. Masters NB, Goodwin Cartwright BM, Rodriguez PJ, Farrar KG, Do D, Stucky NL. Delayed or Absent First Dose of Measles, Mumps, and Rubella Vaccination. JAMA Netw Open. 2026;9(1):e2551814. doi:10.1001/jamanetworkopen.2025.51814
2. Routine Measles, Mumps, and Rubella Vaccination. Centers for Disease Control and Prevention. Updated January 26, 2021. Accessed January 2, 2026. https://www.cdc.gov/vaccines/vpd/mmr/hcp/recommendations.html