As shown in Table 1 , the patients with severe H1N1 were significantly older than those that presented with moderate H1N1 (mean age, 40.1 years vs. 26.1 years; P < 0.01). The mean duration of symptoms before presentation was 4 days, with fever, cough, and myalgia as the most common in both groups. However, the patients with severe H1N1 had more prodromal symptoms (e.g., fever, dyspnea, and myalgia) and co-morbidities [e.g., chronic obstructive pulmonary disease (COPD) and coronary artery disease] than the patients with moderate H1N1 (p < 0.01). Although a relationship with pregnancy was more common in the severe H1N1 group (four patients vs. one patient with moderate H1N1), this difference was not statistically significant (Table 1) . Similarly, there was no difference in the other preexisting diseases such as diabetes mellitus and hypertension between the two groups.