P0499 - Association of Bacteremia with Vaccination Status in Children Aged 2-36 Months

Background
The incidence of bacteremia in young children has decreased since the implementation of routine Haemophilus influenzae type B (HiB) and Streptococcus pneumoniae (S. pneumoniae) vaccinations. The impact of vaccination status on risk of bacteremia in febrile children in the post-vaccine era remains unknown. The objective of this study was to determine the association between bacteremia and vaccination status in children aged 2 to 36 months presenting to a pediatric emergency department (ED).

Methods
This was a retrospective cohort study of children aged 2 to 36 months with blood cultures obtained in the pediatric ED between January 2013 and December 2017. The exposure of interest was immunization status, defined as zero, one, two, or 3+ sets of Hib and S. pneumoniae vaccinations. Main outcome was the growth of pathogenic bacteria on blood culture. Subjects with high-risk medical conditions were excluded. Of 5534 encounters where blood cultures were obtained, a random representative sample of 1000 encounters was reviewed to estimate proportion of those meeting inclusion criteria, demographics, and vaccination status. The results of this sample were extrapolated to all encounters with blood cultures to estimate the rate of bacteremia. Due to the rarity of the outcome, all encounters with positive blood cultures during the study period were reviewed.

Results
Of 5534 encounters, 4738 met inclusion criteria. The incidence of pathogenic bacteremia was 1.7%. The incidence of contaminated blood culture was 5.1% There was no significant difference in the rate of bacteremia between unimmunized children and children with any number of immunizations (1.25% vs 1.79%; RR 0.71, 95% CI 0.36-1.39). The most common pathogens were S. aureus (n=19, 23.5%) and E. coli (n=13, 16.0%). Six children were found to have S. pneumoniae bacteremia and one child had Hib bacteremia; all of these subjects had at least three sets of vaccinations. No vaccine preventable pathogens were isolated from blood cultures of unvaccinated children. We found no S. Pneumoniae or Hib in children 2-6 months of age who were not yet fully vaccinated due to age (95% CI 0-0.13%) and the contamination rate in this group is high compared to children 7-36 mo (6.62% vs. 3.8%).

Conclusion
Bacteremia in young children is an uncommon event. Bacteremia from S. Pneumoniae or Hib is especially uncommon and, in this cohort, was independent of vaccine status. Contaminated blood cultures were much more common than pathogens in all cohorts.