Short-term cardiac autonomic responses to primary immunization in hospitalized term and preterm infants: the prospective PVAD study

Background Hospitalized preterm infants may experience transient apnea, bradycardia, or oxygen desaturation after routine immunization, but whether vaccination is accompanied by a broader alteration in cardiac autonomic regulation remains uncertain. Methods In this prospective single-center observational study, hospitalized term and preterm infants receiving primary hexavalent and 13-valent pneumococcal immunizations underwent continuous electrocardiographic recording from 24 h before to 48 h after vaccination. Linear and nonlinear heart rate variability indices were calculated over six consecutive 12-h windows. Overall temporal changes were assessed using Friedman tests. Prespecified paired comparisons between each post-immunization window and the − 12 to 0 h reference window used Wilcoxon signed-rank tests with Holm-Bonferroni correction. Results Twenty-five infants were enrolled and 21 had analyzable recordings; 18 were born before 32 weeks of gestation. No heart rate variability index showed a significant overall temporal change or a significant difference from the reference window after multiplicity correction. High-frequency power decreased from 85.05 ± 100.39 ms² before immunization to 49.86 ± 46.63 ms² at 0–12 h (adjusted p = 0.5271) and 35.07 ± 24.77 ms² at 12–24 h (adjusted p = 0.1643). Conclusions Primary immunization was not associated with a detectable sustained short-term alteration in cardiac autonomic regulation. These findings are reassuring but do not exclude brief responses or effects confined to clinically vulnerable subgroups.