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

- We propose that increased viral loads are associated with hospitalization.
- Influenza A/H3N2 had the highest viral load of the three viruses.
- Increased influenza A/H3N2 viral loads were not associated with hospitalization.
- High influenza B viral loads are associated with hospitalization/ICU admission.
- Rhinovirus viral loads do not differ by age or patient setting.

BackgroundThe 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.ObjectivesWe 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 designWe 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.ResultsMean viral load (log10 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.ConclusionWhen compared to ambulatory/ER patients, viral load was higher in hospitalized/ICU patients with influenza B, but not influenza A or HRV.