General and health-related backgrounds of study subjects.

*Pandemic period, between April 2009 and July 2010; Post-pandemic period, between August 2010 and the end of survey period.†Socioeconomic level evaluated by ability to pay for utilities, food, and medical service: low income, cannot cover electricity, water, telephone, house rent, foods, any medical service; middle income, can cover a part of electricity, water, telephone, house rent, and foods, but not any medical service; high income, can cover utility and adequate goods, and medical services.‡Number of days from symptom onset to initiation of oseltamivir administration.