Influenza Virus Infection and the Risk of Serious Bacterial Infections in Young Febrile Infants

OBJECTIVE: We aimed to determine the risk of SBIs in febrile infants with influenza virus infections and compare this risk with that of febrile infants without influenza infections.PATIENTS AND METHODS: We conducted a multicenter, prospective, cross-sectional study during 3 consecutive influenza seasons. All febrile infants ≤60 days of age evaluated at any of 5 participating pediatric EDs between October and March of 1998 through 2001 were eligible. We determined influenza virus status by rapid antigen detection. We evaluated infants with blood, urine, cerebrospinal fluid, and stool cultures. Urinary tract infection (UTI) was defined by single-pathogen growth of either ≥5 × 104 colony-forming units per mL or ≥104 colony-forming units per mL in association with a positive urinalysis. Bacteremia, bacterial meningitis, and bacterial enteritis were defined by growth of a known bacterial pathogen. SBI was defined as any of the 4 above-mentioned bacterial infections.RESULTS: During the 3-year study period, 1091 infants were enrolled. A total of 844 (77.4%) infants were tested for the influenza virus, of whom 123 (14.3%) tested positive. SBI status was determined in 809 (95.9%) of the 844 infants. Overall, 95 (11.7%) of the 809 infants tested for influenza virus had an SBI. Infants with influenza infections had a significantly lower prevalence of SBI (2.5%) and UTI (2.4%) when compared with infants who tested negative for the influenza virus. Although there were no cases of bacteremia, meningitis, or enteritis in the influenza-positive group, the differences between the 2 groups for these individual infections were not statistically significant.CONCLUSIONS: Febrile infants ≤60 days of age with influenza infections are at significantly lower risk of SBIs than febrile infants who are influenza-negative. Nevertheless, the rate of UTI remains appreciable in febrile, influenza-positive infants.