The goal of our investigation was to describe the incidence of serious bacterial infection (SBI, defined as bacteremia, urinary tract infection (UTI), or meningitis) in young infants with and without documented viral pathogens.

This was a retrospective cross-sectional study (1/2016–12/2017) in 3 emergency departments (EDs). Previously healthy 0–60-day-old infants were included if at least respiratory viral testing and a blood culture was obtained. The frequency of SBI, the primary outcome, was compared among infants with/without respiratory viral infections using the Pearson Chi-square test (or Fisher's Exact Test) and unadjusted odds ratios (OR).

The median age of the 597-infant cohort was 32 days (interquartile range: 20–45 days); 42% were female. Eighty-three percent were well appearing in the ED and 72% were admitted. ED triage vitals commonly revealed tachypnea (68%), pyrexia (45%), and tachycardia (28%); hypoxemia (5%) was uncommon. Twenty-eight percent had positive viral testing, most commonly RSV (93/169, 55%), parainfluenza (29, 17%), and influenza A (23, 14%). Eighty-three infants (13.9%) had SBI: 8.4% (n = 50) had UTI alone, 2.8% (n = 17) had bacteremia alone, 1.2% (n = 7) had bacteremia + UTI, 1.0% (n = 6) had bacteremia + meningitis, and 0.5% (n = 3) had meningitis alone. Infants with documented respiratory viral pathogens were less likely to have any SBI (OR: 0.23; 95% CI: 0.11–0.50), UTI (OR 0.22, 95% CI: 0.09–0.56), or bacteremia (OR 0.27, 95% CI: 0.08–0.9) than infants with negative viral testing. There was no difference in meningitis frequency based on viral status (OR: 0.13, 95% CI: 0.008–2.25).

The frequency of bacteremia and UTI was lower in young infants with respiratory viral infections compared to infants with negative respiratory viral testing.