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.