MUC5AC and chronic obstructive pulmonary disease: To confirm the observations in the RV challenge model, we performed similar analyses in the naturally occurring COPD exacerbation study and found that sputum MUC5AC concentrations during virus-induced exacerbations also correlated with bacterial loads measured 2 weeks after exacerbation onset, a time point we have previously shown to represent the peak of secondary bacterial infections (ref. 18 and Supplemental Figure 3A).