seasonal outbreak of influenza a virus infection in pediatric age groups during 2004-2005 in south of iran

background: the pandemic and regional influenza outbreaks resulting from antigenic variation of influenza viruses have been the subject of numerous studies which are crucial to the preparation of the vaccine. frequent global winter outbreaks of influenza viruses require a constant surveillance of emerging influenza variants in order to develop efficient influenza vaccine.   methods: this study was conducted from december 2004 to march 2005, during an outbreak of influenza in 300 children of one to 15-year-old. mdck cells grown in microwell plates inoculated with pharyngeal swabs. hemagglutination (ha) of guinea pig erythrocytes by viral isolates was used for initial screening of influenza viruses. the final viral identification was performed by ha inhibition test using reference antisera prepared against who reference antigens.   results: based on the results of ha tests with culture fluid of  mdck inoculated with 300 pharyngeal swabs, 31 (10.3%) of the specimens were suspected to contain influenza viruses of which 10 proved to be either a/h 1 n 1 or a/h 3 n 2 subtypes. by reacting against aforementioned antisera in ha inhibition test. h 3 n 2 subtype prevailed until january 2005 and mostly affected children of younger age, whereas viruses emerging subsequently were only of h 1 n 1 subtype and predominantly isolated from children of older age.   conclusion: a/h 3 n 2 isolate appears to be antigenically different from corresponding reference antigen. this is evident by a 16-fold reduction in antibody titer by using reference antiserum against a/h 3 n 2 isolate (1:8), as compared to that of reference antigen antibody (1:128). iran j med sci 2006; 31(3): 131-134.   keywords ● human influenza ● vaccine ● h3n2 ● h1n1 ● pediatric