Background: Influenza is a highly contagious respiratory disease. Influenza virus, which causes epidemics every winter season, has the high possibility of appearing with new virus types every year due to antigen variation. Therefore, we intended to analyze the data on the epidemiology of influenza that had been acquired by laboratory surveillance in Incheon during the 2003/2004 and 2004/ 2005 seasons and to apply the knowledge to the control and prevention of influenza in Korea.

Methods: Specimens were inoculated into Madin-Darby canine kidney (MDCK) cells and, when cytopathic effect (CPE) was seen, culture supernatants were tested by mutiplex RT-PCR for typing and subtyping of influenza viruses.

Results: The first virus of the season was isolated at week 47 (3rd week on November) in 2003 during 2003/2004 and at week 43 (4th week on October) in 2004 during 2004/2005, which was about 4 weeks earlier than in the 2003/2004 season. From 532 specimens cultured for influenza virus during the 2003/2004 season. 330 (62.0%) viruses were isolated: 161 (48.8%) A/H3N2, 1 (0.3%) A/H1N1, and 168 (50.9%) B. During 2004/2005 season, 457 specimens were tested and 278 (60.8 %) were positive for influenza virus: 232 (83.5%) A/H3N2, 5 (1.8%) A/H1N1, and 38 (13.7%) B. The incidence of influenza was the highest in the school-age children and young adults of 7 to 19 years age group in both seasons.

Conclusion: Influenza virus was isolated at a high rate (more than 60%) by the laboratory influenza surveillance system in Incheon during the 2003/2004 and 2004/2005 seasons: the predominant strain was influenza A/H3N2 subtype. (Korean J Clin Microbiol 2005;8(2):165-171)