Univariate analysis identified statistically significant differences between the severe and mild cases groups in prematurity, age at onset, pertussis vaccination status, IL-5, IL-8, IL-10, TNF-α, IFN-α, IFN-γ, WBC count, neutrophil percentage, platelet count, hemoglobin, length of stay, pneumonia, bronchitis, respiratory failure, encephalopathy, paroxysmal cough, whooping, cyanosis, tachypnea, duration of antibiotic treatment, and usage rates of cefuroxime and amoxicillin (p < 0.05). This evidence concerns the gene IL5 and respiratory failure.