Clinical correlation of influenza and respiratory syncytial virus load measured by digital PCR

Hijano, Diego R.; Brazelton de Cardenas, Jessica; Maron, Gabriela; Garner, Cherilyn D.; Ferrolino, Jose A.; Dallas, Ronald H.; Gu, Zhengming; Hayden, Randall T.

The correlation between viral load and severity of infection remains controversial for respiratory viruses [6, 8, 9, 15, 40] . For RSV, viral load was significantly associated with severity or presence of respiratory illness in children �5 years of age. An analysis of cycle threshold values and disease severity in 4410 patients by Fuller et al. showed that in those with RSV infection, viral load was associated with disease severity only in patients younger than 2 years of age [15] . DeVincenzo et al. reported that patients intubated because of RSV-induced respiratory distress had higher viral loads than those who were not, and that higher viral loads were associated with longer hospitalization [5] . We could not measure viral loads in lower respiratory tract samples in our study. However, a study showed that viral loads in the upper and lower respiratory tracts are strongly correlated when measured at the same time [41] . In an autopsy study of patients with fatal RSV LTRI, the viral antigen was extensively present in the lung tissue [42]. This points to a possible association between viral load and RSV disease severity. However, many studies have not found this association [6, 7] . Gerna et al. reported a direct relationship between viral loads and clinical symptoms in their patient cohort, but viral loads were not associated with an increased risk of developing bronchiolitis [7] .