Febrile Infants: Bacterial, Viral, or Both?

William T. Basco, Jr., MD, MS


March 15, 2019

The Risk for Bacterial-Viral Coinfection

Past evidence has suggested that febrile children with viral infections are at lower risk for serious bacterial infection (SBI). However, much of those data were retrospective in nature, came from single-center studies, or were conducted as long as 10 years ago. In a recent report, Mahajan and colleagues[1] performed a planned secondary analysis of data collected prospectively at 26 pediatric emergency departments participating in a national research network.

The analysis cohort included almost 3000 febrile children (temperature at least 38º C) aged 0-60 days who had laboratory testing for SBI (blood, urine, and/or cerebrospinal fluid cultures) and at least one viral test. Premature infants and those with obvious sepsis or other complicating diagnoses were excluded. Because of the prospective nature of the data, each site used the same observation score to assess clinical illness. Approaches to testing and collection and reporting of demographic and clinical variables also were similar among the study sites.

SBI was defined as the presence of bacterial meningitis, bacteremia, urinary tract infection (UTI), or any combination of these three infections. The mean age of the children was 34 days, and 37.1% of the infants were ≤ 28 days old.

Overall, just under 1 out of 10 of these infants had an SBI (urinary tract infections: 7.4%; bacteremia: 2%; meningitis: 0.6%). The risk for a bacterial infection according to viral testing status is shown in the Table.

Table. Risk for Bacterial Coinfection

Infection Viral-Negative Viral-Positive
All SBI 12.7% 3.7%
UTI 10.7% 2.8%
Bacteremia 2.9% 0.8%
Meningitis 0.8% 0.4%

When looking at the bounds of the 95% confidence intervals among the viral-positive children, for meningitis the range was 0.1%-1.0%. For bacteremia, the range was 0.3%-1.4%. In general, any of the SBIs were more prevalent in the children aged ≤ 28 days compared with those aged 28 days or older. The authors concluded that febrile infants aged 0-60 days with viral infections have lower rates of SBI, but the risk for bacteremia or meningitis may be in the 1% range.


The authors are correct that how clinicians can apply these findings to everyday practice is not straightforward. Their main point is that "lower risk" does not equal "zero risk," so decisions still have to be made at the individual patient level.

Remembering that infants aged 28 days and younger are a different group from infants in the second month of life is very important. Also, knowing that the risk for bacteremia or meningitis is in the 1% range may also be important if the child has been difficult to obtain specimens from or there are other factors precluding obtaining samples.

So, the viral status should be one factor, but not the only factor, that plays into the decision-making for whom to test or treat.

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