Abstract
BACKGROUND: Addressing Missed Opportunities for Vaccination (MOV) contributes to increased vaccination rates in children, reinforcing the need to investigate and intervene in the related factors. OBJECTIVE: To analyze factors associated with missed opportunities for vaccination in children under one year of age in a Brazilian capital. METHODS: This was a cross-sectional, analytical study conducted in seven Basic Health Units in Teresina, Piauí, Brazil. A previously validated questionnaire was applied to parents or guardians of a sample of 316 children. Data were collected from March to June 2025. Multivariable Logistic Regression was performed, and results were expressed as Odds Ratios. RESULTS: Among the children, 53.5% had at least one MOV. The associated factors were: parents with two or more children (95% CI: 1.06-2.96), false contraindications (95% CI: 1.29-8.73), inadequate assessment of vaccination cards by health professionals (95% CI: 1.78-29.00), vaccine shortages in health units (95% CI: 1.57-18.28), and refusal to open multidose vaccine vials (95% CI: 1.81-19.31). Receiving information about vaccination in the previous month was a protective factor against MOV (95% CI: 0.25-0.77). The vaccines most frequently contributing to MOV were BCG (15.8%) and the COVID-19 vaccine, with 15.5% for the first dose and 14.9% for the second. CONCLUSIONS: The high prevalence of MOV found in this study indicates weaknesses in the immunization process and suggests the need for implementing measures to interrupt the chain of causes leading to MOV, thereby contributing to the achievement of the objectives of the Brazilian National Immunization Program.