Influenza and rhinovirus viral load and disease severity in upper respiratory tract infections

[[[ Background: ]]] The role of viral load in respiratory viral infection is unclear. It is proposed that the viral load of some, but not all respiratory viruses correlate with disease severity. [[[ Objectives: ]]] We aimed to determine if an association exists between viral loads among patients in ambulatory settings, compared to those requiring hospitalization/intensive care unit (ICU) admission with influenza A/H3N2, influenza B, or human rhinovirus (HRV); we also explored the impact of age, gender and co-detection of Streptococcus pneumoniae on patient setting. We hypothesized that hospitalized/ICU patients have higher respiratory virus viral loads compared to ambulatory (e.g. walk-in clinics, family practices)/ER patients. [[[ Study design: ]]] We quantified viral load by in-house real-time RT-PCR in 774 nasopharyngeal swabs with influenza A/H3N2, or B or HRV viruses from various patient settings in Ontario, Canada. [[[ Results: ]]] Mean viral load (log_{10} copies/ml) of influenza A/H3N2 (6.94) was higher than influenza B (4.96) and HRV (5.58) (p<0.0001). Influenza A/H3N2 viral loads were highest in infants and the elderly; however, increased A/H3N2 viral loads were not associated with hospitalization/ICU admission compared to swabs collected in ambulatory/ER settings. Influenza B viral loads were higher in patients in hospital/ICU settings compared to those in ambulatory settings (OR 1.28, 95% CI 1.11-1.47). HRV viral loads did not differ by age (p=0.67) or setting (p=0.54); there was no association between S. pneumoniae colonization and setting for any virus. [[[ Conclusion: ]]] When compared to ambulatory/ER patients, viral load was higher in hospitalized/ICU patients with influenza B, but not influenza A or HRV.