Definitions
Acute respiratory illness (ARI) was defined as a new onset of cough or worsening of a chronic cough. Severe ARI (SARI) was defined as an illness requiring hospitalization with measured axillary temperature ≥38°C plus a new onset of a cough, worsening of a chronic cough, or difficulty in breathing. Cardiopulmonary condition was defined as the presence of any known physician-diagnosed chronic cardiovascular conditions (such as coronary artery diseases, cardiac arrhythmias, valvular heart diseases, cardiomyopathy) and/or any chronic lung diseases (such as chronic obstructive pulmonary disease (COPD), asthma, chronic bronchitis, pulmonary tuberculosis, emphysema, and lung fibrosis).