The specificity and the quality of the immune response against novel strains of influenza is often diminished in individuals who are repeatedly immunized (by vaccination or recurrent infections). However, the impact of antigenic sin on protection has not been well established, and appears to differ with each infectious agent vaccine, geographic location, and age. Researchers found reduced antibody responses to the 2009 pandemic H1N1 influenza vaccine in individuals who had been vaccinated against the seasonal A/Brisbane/59/2007 (H1N1) within the previous three months.