Serious bacterial infection in recently immunized young febrile infants.

OBJECTIVE: The objective of this study was to investigate the prevalence of serious bacterial infection (SBI) in febrile infants without a source aged 6-12 weeks who have received immunizations in the preceding 72 hours.

METHODS: The authors conducted a medical record review of infants aged 6-12 weeks with a fever of > or = 38.0 degrees C presenting to the pediatric emergency department (ED) over 88 months. Infants were classified either as having received immunizations within the 72 hours preceding the ED visit (recent immunization [RI]) or as not having received immunizations during this time period (no recent immunization [NRI]). Primary outcome of an SBI was based on culture results; only patients with a minimum of blood and urine cultures were studied.

RESULTS: A total of 1,978 febrile infants were studied, of whom 213 (10.8%) had received RIs. The overall prevalence of definite SBI was 6.6% (95% confidence interval [CI] = 5.5 to 7.7). The prevalence of definite SBI in NRI infants was 7.0% (95% CI = 5.9 to 8.3) compared to 2.8% (95% CI = 0.6 to 5.1) in the RI infants. The prevalence of definite SBI in febrile infants vaccinated in the preceding 24 hours decreased to 0.6% (95% CI = 0 to 1.9). The prevalence of definite SBI in febrile infants vaccinated greater than 24 hours prior to presentation was 8.9% (95 CI = 1.5 to 16.4). The relative risk of SBI with RI was 0.41 (95% CI = 0.19 to 0.90). All SBIs in the RI infants were urinary tract infections (UTI).

CONCLUSIONS: Among febrile infants, the prevalence of SBI is less in the initial 24 hours following immunizations. However, there is still a substantial risk of UTI. Therefore, urine testing should be considered in febrile infants who present within 24 hours of immunization. Infants who present greater than 24 hours after immunizations with fever should be managed similarly to infants without RIs.