Immunization of extremely low birth weight (ELBW) infants in the neonatal intensive care unit (NICU) is associated with adverse events including fever and apnea/bradycardia in the immediate post-immunization period. This presents a diagnostic dilemma for clinicians, leading to the potential for immunization delay and sepsis evaluations. The goal of this study is to compare the incidence of sepsis evaluations, need for increased respiratory support, intubation, seizures, and death among immunized ELBW infants in the 3 days pre- and post-immunization.