Respiratory Syncytial Virus Load Predicts Disease Severity in Previously Healthy Infants

BACKGROUND: Elucidating the relationship between viral load and respiratory syncytial virus (RSV) disease severity is critical to understanding pathogenesis and predicting the utility of antivirals. METHODS: Previously healthy, naturally RSV-infected infants <24 months old not treated with ribavirin, passive antibody, or corticosteroids were prospectively studied (n=141). Viral loads were measured by fresh quantitative culture from nasal washes collected at a single time point shortly after hospitalization. RESULTS: The subjects' mean age was 112.1 days, and the mean estimated gestational age at birth was 38.38 weeks. RSV load decreased with longer durations of symptoms before specimen collection (P=.01). Male subjects had higher RSV loads than female subjects (P=.036). Significant independent predictors of longer hospitalization were congenital anomaly (P<.0001), lower weight on admission (P=.028), and higher nasal RSV load (P=.008). A 1-log higher RSV load predicted a 0.8-day longer hospitalization. Lower weight and higher RSV load were also independently associated with respiratory failure (P<.0005 and P=.0049, respectively) and requirement for intensive care (P=.0007 and P=.0048, respectively). CONCLUSIONS: In previously healthy infants, higher RSV loads measured at capturable time points after symptom onset predict clinically relevant measures of increased disease severity.