One infant in the Neonatal Intensive Care Unit (NICU) at St. Louis Children's Hospital experienced respiratory decompensation and tested positive for influenza virus type A by fluorescent antibody stain performed on a nasopharyngeal swab. This infant received treatment doses of oseltamivir. Subsequently, 27 other infants received oseltamivir prophylaxis for exposure to influenza virus type A. Exposed infants were those who shared a primary medical team, nursing care, respiratory therapist, physical therapist, or occupational therapist with the influenza A positive infant. Prophylaxis was deemed necessary by the attending neonatologist after consultation with the Infectious Diseases Division of the Department of Pediatrics at the Washington University School of Medicine.

Treatment dose was oseltamivir 3 mg/kg/dose by mouth (PO) twice daily. Prophylactic dose was oseltamivir 1 mg/kg/dose PO once daily to infants < 28 weeks postmenstrual age (PMA), 1 mg/kg/dose PO twice daily to infants 28 - 38 weeks PMA, and 3 mg/kg/dose PO once daily to infants > 38 weeks PMA. Dosing in infants < 28 weeks PMA was chosen based on unpublished data from Acosta et al. This data was obtained from phone contact with Dr. Peter Gal, co-author of the study. Dosing in infants 28 - 38 weeks PMA was chosen based on published data from Acosta et al.1 Dosing in infants > 38 weeks PMA and less than 3 months postnatal age was chosen based on data from Kimberlin et al. Dosing in infants > 38 weeks PMA and greater than 3 months postnatal age was per the recommendations of the Advisory Committee on Immunization Practices of the United States Department of Health and Human Services.

Pharmacokinetic parameters calculated for individual subjects will include apparent distribution volume and elimination half-life of oseltamivir (OST) and OST carboxylate (CBX). Area under the plasma concentration versus time curve (AUC) of OST and OST CBX will be determined using the trapezoidal method. A population pharmacokinetic model will be generated based on pooled pharmacokinetic data (2-compartment model for OST and 1-compartment model for OST CBX) utilizing maximum likelihood estimation. A weight-based dosing table will be generated to provide AUC exposures comparable to adults.

OST target level attainment of the currently recommended dosing regimen in neonates and infants will be calculated. Currently recommended doses were utilized for all neonates and infants in our cohort. The target level attainment in this cohort will be calculated.