Reinfection with antigenically similar influenza virus observed at a pediatric clinic in Osaka from December 1998 to April 2002

Abstract In the past 6 years, influenza A viruses with less antigenic variations have been circulating throughout the world. Influenza B viruses with similar antigenicity were also isolated from 1998 to 2001 in Japan. We isolated 1294 influenza viruses from 3336 patients displaying influenza-like illness at a pediatric clinic during four influenza seasons. Among them, 17 cases of reinfection with the same sero/subtype of influenza virus were confirmed. These cases included three cases of influenza A (H1N1) virus, seven cases of influenza A (H3N2) virus and seven cases of influenza B virus. First infection and reinfection strains from the same patients were genetically similar, but not identical. Each virus involved in a reinfection case seemed to relate closely to epidemic strains spreading at that time. The hemagglutinination inhibition (HI) titer to vaccine strains did not rise in 13 of the 14 patients' sera, obtained after first infection or just before the reinfection. These data show that the reinfection with antigenically similar influenza virus can occur in infants, although the frequency of it seems low. In our studies, weak immunological response, not slight variation on influenza virus, was shown to be the primary condition for the reinfection to occur.