Influenza viruses are major respiratory pathogens that cause seasonal epidemics and occasional pandemics. Immune response to influenza viruses is majorly targeted against the hemagglutinin antigen. A laboratory-based retrospective cross-sectional study was conducted on 50 acute and 50 follow-up samples to assess the immune response to influenza A(H1N1)pdm09 virus after natural infection and detect the presence of pre-existing antibodies against influenza A(H3N2) and influenza B viruses. Two-fourfold rise in hemagglutination-inhibition (HAI) titer was observed in 100% of the follow-up samples for influenza A(H1N1)pdm09 virus. No change in HAI titers for influenza A(H3N2) and influenza B viruses was observed.
Keywords: hemagglutination-inhibition assay; immune response; influenza virus; pre-existing antibodies.