Preterm Infants Are Undervaccinated at 19 and 36 Months

Infant resting in NICU
Infant resting
Preterm infants are undervaccinated at 19 and 36 months when compared with infants born at term or postterm.

In the United States, approximately 10% of infants are born prematurely, defined as born at <37 weeks gestation. Preterm infants are at increased risk for vaccine-preventable infections and associated complications. Routine vaccination of these high-risk infants is recommended by the US Advisory Committee on Immunization Practices (ACIP) based on evidence that vaccines induce protective immunity and are safe and well tolerated in this population. Medically stable preterm infants should be vaccinated at the same chronologic age and follow the same vaccine schedule as infants born at term, with the exceptions of hepatitis B and rotavirus vaccines. Previous studies have found that preterm infants are at an increased risk for delayed vaccination and undervaccination, but studies assessing timely receipt of all recommended vaccines are limited. Therefore, this retrospective cohort study compared early childhood vaccination among preterm infants with that of term/postterm infants born between 2008 and 2013.

In total, 10,367 infants born in the state of Washington with birth hospitalizations at an urban academic medical center were included. Of these infants, 19.3% were born prematurely. Vaccine data were obtained from the Washington State Immunization Information System, and demographic, clinical, and visit data from electronic health records were linked. The primary outcome was completion of the recommended 7-vaccine series by age 19 months compared between preterm infants born at <37 weeks’ gestation and term/postterm infants born between 37 and 43 weeks’ gestation. This outcome was calculated using Pearson’s X2 test and multivariable logistic regression. The secondary outcomes included 7-vaccine series completion by 36 months of age and receipt of individual vaccines in the series. Vaccination for rotavirus, hepatitis A, and influenza were also assessed.

Results showed that a significant percentage of preterm infants were undervaccinated by 19 months of age and failed to catch up by 36 months of age. When compared with term/postterm infants, preterm infants had lower 7-vaccine series completion by 19 months (54.0% vs 47.5%) and 36 months (71.3% vs 63.6%). Further, early preterm (defined as born at 23-33 weeks gestation) and late preterm (defined as born at 34-36 weeks gestation) infants had lower rates of 7-vaccine series completion compared with term/postterm infants. Full influenza vaccination coverage by age 19 months also differed between groups: 47.7% of early preterm infants, 41.5% of late preterm infants, and 44.7% of term/postterm infants had completed the series (P =.02).

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Possible reasons for undervaccination of preterm infants at age 19 months and 36 months remain unclear but could reflect provider and parental factors that include vaccine safety beliefs and vaccine communication between the provider and family. Overall, the study authors concluded that, “Future work is needed to inform the design and implementation of interventions aimed at improving timely vaccination coverage of these high-risk infants.”


Hofstetter AM, Jacobson EN, DeHart P, Englund JA. Early childhood vaccination status of preterm infants. Pediatrics. 2019;144(3):e20183520.