The explosive increase in the infection rate during two months (mid-October to mid-December) of the study period may be a typical pattern of spread of an acute respiratory viral infectious disease that has a short incubation period (1-3 days). This pattern is similar to those reported in other regions of Korea and other countries [3, 4, 17] . Vaccination against the 2009 H1N1 influenza virus in Korea started on 18 November, 2009 for school-aged children, on 7 December, 2009 for children aged 6 months to 5 years and on 21 December, 2009 for adults with risk factors, which was after the peak of the epidemic. The rapid disappearance of the spread of infection might be the effect of vaccination and the strengthened individual hygiene, but the epidemiologic pattern suggests that an unknown herd immunity against a new viral infection might also be responsible [3, 17] .