Background. The evaluation of young febrile infants is controversial, in part because it is unclear whether clinical evidence of a viral infection significantly reduces the risk of serious bacterial infections (SBIs). Specifically, it remains unclear whether the risk of SBI is altered in a meaningful way in the presence of respiratory syncytial virus (RSV) infections.

Objective. The objective of this study was to determine the risk of SBI in young febrile infants who are infected with RSV compared with those without RSV infections.

Methods. We conducted a 3-year multicenter, prospective, cross-sectional study. All febrile (≥38°C) infants who were ≤60 days of age and presented to any of 8 pediatric emergency departments from October through March 1998–2001 were eligible. General clinical appearance was evaluated using the Yale Observational Scale. We determined RSV status by antigen testing of nasopharyngeal secretions. We defined bronchiolitis as either wheezing alone or chest retractions in association with an upper respiratory infection. We evaluated infants with blood, urine, cerebrospinal fluid, and stool cultures. Urinary tract infection (UTI) was defined by single pathogen growth of ≥5 × 104 cfu/mL, or ≥104 cfu/mL in association with a positive urinalysis in a catheterized specimen, or ≥103 cfu/mL in a suprapubic aspirate. Bacteremia, bacterial meningitis, and bacterial enteritis were defined by growth of a known bacterial pathogen. SBI was defined as any of the above-mentioned 4 bacterial infections.

Results. We enrolled 1248 patients, including 269 (22%) with RSV infections. The overall SBI status could be determined in 1169 (94%) of the 1248 patients, and the rate of SBIs was 11.4% (133 of 1169; 95% confidence interval [CI]: 9.6%–13.3%). The rate of SBIs in the RSV-positive infants was 7.0% (17 of 244; 95% CI: 4.1%–10.9%) compared with 12.5% (116 of 925; 95% CI: 10.5%–14.8%) in the RSV-negative infants (risk difference: 5.5%; 95% CI: 1.7%–9.4%). The rate of UTI in the RSV-positive infants was 5.4% (14 of 261; 95% CI: 3.0%–8.8%) compared with 10.1% (98 of 966; 95% CI: 8.3%–12.2%) in the RSV-negative infants (risk difference: 4.7%; 95% CI: 1.4%–8.1%). The RSV-positive infants had a lower rate of bacteremia than the RSV-negative infants (1.1% vs 2.3%; risk difference: 1.2%; 95% CI: −0.4% to 2.7%). No RSV-positive infant had bacterial meningitis (0 of 251; 95% CI: 0%–1.2%); however, the differences between the 2 groups with regard to bacteremia and bacterial meningitis did not achieve statistical significance.

Conclusions. Febrile infants who are ≤60 days of age and have RSV infections are at significantly lower risk of SBI than febrile infants without RSV infection. Nevertheless, the rate of SBIs, particularly as a result of UTI, remains appreciable in febrile RSV-positive infants.

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