RESUMEN
The global population has been exposed to the novel pandemic H1N1 influenza virus since mid March 2009, causing the expansion of respiratory illness around the world, including Thailand. To evaluate the an-tibody titers against human pandemic influenza (H1N1) in Thai people with influenza-like illness (ILI), 45 paired se-rum samples (acute and convalescent) were subjected to hemagglutination inhibition (HI) test and real-time RT-PCR. Most serum samples of ILI patients positive by real-time RT-PCR displayed an at least four-fold antibody in-crease of HI titers against pandemic influenza (H1N1). In addition, to determine cross-reactivity with human sea-sonal H1N1 influenza, viral antigen from the seasonal H1N1 was used to detect antibody against seasonal H1N1 influenza and all sera showed negative results. We also studied the single sera samples from the high risk medical personals collected before and after the pandemic influenza (H1N1) outbreaks for antibodies against seasonal H3 influenza virus infection. The results showed lack of cross-reactivity to the human pandemic H1N1 influenza virus. HI antibody testing to pandemic influenza (H1N1) can be used for the diagnosis, preventive and control measures of potential outbreaks.