Preterm and small-for-gestational-age (SGA) infants are more susceptible to vaccine-preventable diseases. To evaluate routine vaccination timeliness in these high-risk groups, a full birth cohort of infants (n = 41,502) born in 2017 and 2018 in Tuscany was retrospectively followed up until 24 months of age. Infants were classified by gestational age (GA) and SGA status. The vaccinations included: hexavalent (HEXA), measles-mumps-rubella, varicella, pneumococcal conjugate (PCV), and meningococcal C conjugate. Time-to-event (Kaplan–Meier) analyses were conducted to evaluate the timing of vaccination according to GA; logistic models were performed to evaluate the associations between GA and SGA with vaccination timeliness. Time-to-event analyses show that the rate of delayed vaccine receipt increased with decreasing GA for all the vaccinations, with a less marked gradient in later vaccine doses. Compared to full-term infants, very preterm infants significantly showed an increased odds ratio (OR) for delayed vaccination in all the vaccinations, while moderate/late preterm infants only showed an increased OR for HEXA-1, HEXA-3, PCV-1, and PCV-3. SGA infants had a significantly higher risk of delayed vaccination only for HEXA-1 and PCV-1 compared to non-SGA infants. In conclusion, vaccinations among preterm and SGA infants showed considerable delay. Tailored public health programs to improve vaccination timeliness are required in these high-risk groups.
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