Pandemic influenza A(HlNl)2009 (pHlNl 2009) was firstly occurred in the Northern area in Vietnam from 08th June, 2009. With the application of descriptive -epidemiological study method, the study aimed to describe the epidemical development and epidemiology of pHlNl 2009 confirmed cases in the Northern area of Vietnam within 2 years, from June, 2009 to June, 2011. The results showed that pHlNl 2009 occurred in the area by imported cases in big and crowded cities. After 6 weeks, pHlNl 2009 was transmited in the communities and expanded to all cities and provinces in the ares within 5 months. There were 2 epidemical waves: The first one was huger, lasted for 7 months (from June, 2009 to January, 2010) with the peak in the late September to October, 2009
The latter one was smaller, lasted for 5 months (from November, 2010 to April, 2011). The changes in the partent of cases were observed in the comparission with the seasonal influenza: nearly all (99 percent) of cases due to pHlNl 2009 were happened in groups younger than 55 years old, among whom the group aged from 15 to 24 was most affected, being account for 47.9 percent. Additionally, almost (92.9 percent) of cases reported was young and not in the high risk group. The study provided scientific information essential to develop an effective epidemic controlling strategy for coming Influenza pandemic that we have to face in the future.