Influenza Virus Infection in Infants Less than Three Months of Age

From The Pediatric Infectious Disease Journal®

Bender, Jeffrey M. MD; Ampofo, Krow MD; Gesteland, Per MD, MSc; Sheng, Xiaoming PhD; Korgenski, Kent MS; Raines, Bill; Daly, Judy A. PhD; Valentine, Karen MS; Srivastava, Rajendu MD, FRCP(C), MPH; Pavia, Andrew T. MD; Byington, Carrie L. MD

Abstract
Objective: We evaluated the presentation, outcomes, and the risk of serious bacterial infection (SBI) in infants < 3 months old with influenza virus infection.

Patients and Methods: We identified demographic, hospitalization, and microbiologic data from computerized medical records for all infants and children < 24 months of age, with laboratory confirmed influenza infection cared for at a tertiary care children's hospital during 4 winter seasons (2004–2008). We compared those < 3 months of age with older groups.

Results: We identified 833 children < 24 months of age with laboratory-confirmed influenza. Of those, 218 were < 3 months old. Influenza accounted for 3.6% of all evaluations of febrile infants and 12% of febrile infant encounters during winter. Infants < 3 months of age were less likely to have a high risk chronic medical condition, but were more likely to be hospitalized than children 3 to < 24 months old (P < 0.005). Infants < 3 months with influenza had fewer prolonged hospital stays than those 3 to < 6 months old [P = 0.056; OR: 0.5 (0.24–1.0)] and 6 to < 12 months old [P = 0.011; OR: 0.43 (0.24–0.83)]. Five (2.3%) infants < 3 months old had SBI.

Conclusions: Infants < 3 months of age with influenza virus infection often present with fever alone. Although they are more likely to be hospitalized than those 3 to < 24 months old, hospital stays are short and outcomes generally good. Infants with influenza virus infection have a low risk of concomitant SBI.

Introduction
Febrile infants younger than 3 months are often evaluated for serious bacterial infection (SBI). Approximately 8.5% to 9.5% of these infants have a SBI, with the remaining fevers presumably caused by viral illness.[1,2] Many studies have demonstrated that febrile infants with the clinical diagnosis of a specific viral illness such as bronchiolitis or a laboratory confirmed diagnosis of enterovirus or respiratory syncytial virus (RSV), are at lower risk for SBI than those with otherwise undifferentiated fever.[2–11] There are few data on the risk of SBI and outcomes of young infants with laboratory-confirmed influenza.

Influenza is a common viral cause of fever in young children during the winter.[12,13] Unlike other respiratory viruses, influenza frequently presents with high fever[14] that can be difficult to differentiate from SBI. In infants younger than 3 months, this often results in an evaluation for SBI and hospitalization.

Influenza vaccination is recommended for infants and children 6 months and older. Recent studies have demonstrated significant morbidity and mortality associated with influenza infection in infants younger than 6 months who are too young to be immunized.[15–17] Likewise, influenza infection in infants 6 months and younger may lead to high hospitalization rates and associated hospital costs.[17–19]

The objective of this study was to describe the outcomes, including rates of concomitant SBI, of a large cohort of infants younger than 3 months of age with laboratory confirmed influenza. We compare the outcomes of this group of infants with the outcomes among infants and children 3 to < 24 months old.

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