Abstract
INTRODUCTION: The 2009 swine-origin H1N1 influenza virus (swH1N1) provided an opportunity to study immune responses to a new influenza strain in the context of seasonal influenza vaccination. Our goals were: to assess whether analyzing multiple parameters of immune responsiveness to influenza has an advantage over evaluating hemagglutination inhibition (HAI) titer alone, to determine whether vaccination with the seasonal vaccine induced cross-reactive immunity to swH1N1 in some individuals, and to determine whether the immune response against swH1N1 is higher after infection than vaccination. METHODS: Antibody and T cell responses were studied in ten subjects who were first immunized with the 2009-2010 seasonal influenza subunit vaccine, then 6 weeks later with the swH1N1 monovalent subunit vaccine. The amount of antibody against native virus glycoproteins, overall avidity of these antibodies, and HAI titer were measured. T cells were evaluated for proliferation and IFNγ secretion in response to the vaccine in vitro. Individuals with influenza-like illness were also evaluated, adding a microplate neuraminidase inhibition (NAI) test. RESULTS: The immune response to influenza was highly variable and immune parameters did not increase in parallel. The seasonal vaccine induced antibodies recognizing the pandemic virus in 50% of subjects. Antibody affinity and NAI activity to swH1N1 were higher after natural infection than vaccination. CONCLUSIONS: The evaluation of several immune parameters gives a more complete measure of immune responsiveness to influenza infection or vaccination than the HAI test alone.